La brioche è più dolce dei cracker, ma di mattina mi piacciono sia la brioche sia i cracker.

Italian often uses the definite article with foods and other nouns when speaking about them in a general sense.

So:

La brioche è più dolce dei cracker.

literally looks like The brioche is sweeter than the crackers, but in natural English the meaning is more like Brioche is sweeter than crackers.

This is very common in Italian:

• Il pane è buono. = Bread is good.
• La pasta mi piace. = I like pasta.

So the article does not always mean a specific single brioche.

Yes. In standard Italian, brioche is normally treated as feminine:

• la brioche
• una brioche

So the sentence correctly says La brioche è...

Also, brioche is usually unchanged in the plural too:

• la brioche
• le brioche

Because è is the verb is from essere.

Italian uses the accent to distinguish it from e, which means and.

• è = is
• e = and

So:

• La brioche è dolce. = The brioche is sweet.
• La brioche e i cracker = The brioche and the crackers

Because dolce agrees with la brioche, which is singular.

In the first clause, the thing being described is la brioche, so the adjective is singular:

• La brioche è più dolce...

If the subject were plural, you would use dolci:

• Le brioche sono dolci.

Also, adjectives ending in -e have the same form for masculine and feminine singular:

• dolce = singular masculine or feminine
• dolci = plural masculine or feminine

Because di is used in this kind of comparison, and di + i contracts to dei.

So:

• di + i cracker → dei cracker

That gives:

• più dolce dei cracker = sweeter than crackers

This is a very common pattern:

• più alto di Marco = taller than Marco
• più veloce della macchina = faster than the car
• più dolce dei cracker = sweeter than the crackers / crackers

In Italian, both di and che can appear in comparisons, but they are used in different situations.

Use di most commonly when comparing:

• one noun with another noun
• one pronoun with another
• quantities or similar noun-based comparisons

So here:

• La brioche è più dolce dei cracker.

That is the normal choice because brioche is being compared with cracker.

Very roughly:

• di is common with noun-to-noun comparison
• che is common in other structures, such as comparing two qualities, two verbs, or after certain words

Examples:

• Sono più alto di Luca.
• È più facile che utile.
• Mi piace più leggere che scrivere.

Because cracker is a borrowed word, and many borrowed words in Italian do not change form in the plural.

So:

• il cracker = the cracker
• i cracker = the crackers

The article shows the number:

• il tells you it is singular
• i tells you it is plural

This is common with many loanwords ending in a consonant.

Di mattina means in the morning or in the mornings, depending on context.

In this sentence:

• ma di mattina mi piacciono... means
• but in the morning / in the mornings I like...

It is an idiomatic time expression. Italian also has other common ways to say this:

• al mattino
• la mattina

All can mean in the morning, though the exact tone or regional preference may vary.

Because piacere works differently from English to like.

Italian structure is more like:

• something is pleasing to me

So:

• mi piace la brioche = I like brioche literally: brioche is pleasing to me

In your sentence, the things that are pleasing are la brioche and i cracker, so the verb must match them, not me.

That is why Italian says:

• mi piacciono sia la brioche sia i cracker

not:

• io piaccio...
  because piaccio means I am pleasing / people like me

Because the subject of the verb is plural overall:

sia la brioche sia i cracker

This means both brioche and crackers, so together they form a plural subject. Therefore the verb is plural:

• mi piacciono = I like them / they are pleasing to me

Compare:

• Mi piace la brioche. = one thing
• Mi piacciono i cracker. = more than one thing
• Mi piacciono la brioche e i cracker. = two items together, so plural

Sia... sia... means both... and...

So:

• sia la brioche sia i cracker = both brioche and crackers

It is a very common paired structure in Italian.

Examples:

• Mi piacciono sia il tè sia il caffè. = I like both tea and coffee.
• Parla sia inglese sia italiano. = He/She speaks both English and Italian.

It is slightly more formal or careful than simply saying e, but very common and natural.

Because in Italian, when you link two nouns with sia...sia..., each noun normally keeps its own article if it would normally have one.

So:

• sia la brioche sia i cracker

not usually:

• sia brioche sia cracker

The articles make the phrase sound complete and natural. Italian generally prefers to repeat them in this kind of structure.

Because mi is an unstressed object pronoun, and these pronouns usually come before the conjugated verb in normal statements.

So:

• mi piacciono
• ti piace
• gli piacciono

Here:

• mi = to me
• piacciono = are pleasing

So the order is the standard one:

• di mattina mi piacciono...

Yes, but it changes the emphasis.

• mi piacciono = normal, neutral
• a me piacciono = I like them / as for me, I like them

You can even use both together for emphasis:

• A me mi piacciono sia la brioche sia i cracker.

But in standard careful Italian, the most neutral form is simply:

• mi piacciono sia la brioche sia i cracker

So a me is possible when you want contrast or emphasis, but it is not necessary here.