Prima di infornare la torta, metto l’impasto nello stampo rotondo.

Because after prima to mean before doing something, Italian normally uses di + infinitive.

So:

• prima di infornare = before baking / before putting in the oven
• prima di uscire = before going out
• prima di mangiare = before eating

Without di, it would sound incomplete here.

Infornare literally means to put into the oven. In cooking contexts, it often corresponds to to bake or to put in the oven.

So infornare la torta is literally to put the cake into the oven, but in natural English it may simply be translated as to bake the cake.

It is more specific than cuocere, which means to cook in a broader sense.

Italian usually drops the subject pronoun when it is already clear from the verb ending.

• metto already means I put
• metti = you put
• mette = he/she puts

So io is not necessary unless you want emphasis or contrast:

• Io metto l’impasto, tu prepari il forno.
  = I put in the batter, you prepare the oven.

It is the result of elision.

The full form is:

• lo impasto

But Italian normally shortens lo before a vowel:

• l’impasto

This is very common:

• l’acqua
• l’olio
• l’amico

So l’impasto means the batter / the dough / the mixture.

Because stampo takes the article lo, not il.

In Italian, masculine singular nouns beginning with:

• s + consonant
• z
• ps
• gn
• x
• y

often use lo.

Since stampo begins with st-, we get:

• lo stampo = the tin / mold / cake pan

And in + lo contracts to:

• nello

So:

• nello stampo = in the tin / in the pan

Compare:

• nel forno from in + il forno
• nello stampo from in + lo stampo

In this sentence, stampo means a baking tin, cake pan, or mold.

It is the container you pour the batter into before baking.

So nello stampo rotondo means into the round cake pan / round mold.

In Italian, adjectives often come after the noun.

So:

• stampo rotondo = round pan
• torta grande = big cake
• forno caldo = hot oven

Sometimes adjectives can come before the noun, but the normal, straightforward position is often after it.

Because rotondo agrees with stampo, not with torta.

• stampo is masculine singular
• so the adjective must also be masculine singular: rotondo

If it described torta, it would be:

• torta rotonda

Agreement in Italian depends on the noun being described.

Italian uses definite articles more often than English.

So it is very normal to say:

• la torta
• l’impasto
• lo stampo

Even where English might use no article or a different structure.

In this sentence, the articles make the whole process sound natural and specific: the cake, the batter, the pan involved in the recipe.

They refer to different stages of the same thing.

• l’impasto = the batter / mixture / dough before baking
• la torta = the cake as the final item

So the sentence says that before baking the cake, the speaker puts the batter into the round pan.

That sequence is logical: first the batter goes into the pan, then the cake goes into the oven.

Italian often uses the present tense to describe:

• habitual actions
• recipe steps
• general procedures
• what someone usually does

So metto l’impasto nello stampo rotondo can mean:

• I put the batter into the round pan
• I put the batter into the round pan as part of the recipe / procedure

This is very natural in instructions and descriptions of cooking.

The comma is natural and helpful because the sentence begins with an introductory phrase.

• Prima di infornare la torta, metto l’impasto nello stampo rotondo.

It separates the before... part from the main action.

In some short sentences, punctuation can vary, but here the comma is standard and makes the sentence easier to read.

Yes, Italian is somewhat flexible, though this version is very natural.

For example, you could also say:

• Metto l’impasto nello stampo rotondo prima di infornare la torta.

This means the same thing.

Starting with Prima di... puts emphasis on the time sequence: before baking the cake.

Yes. Impasto is a broad word and can mean:

• dough
• batter
• mixture

The exact translation depends on the recipe.

For a cake, English often prefers batter or cake mixture, but impasto is the normal Italian word for the prepared mixture before baking.

Yes, that is a very common pattern.

If the same person does both actions, Italian usually uses:

• prima di + infinitive

Example:

• Prima di uscire, chiudo la finestra.
  = Before going out, I close the window.

If the subject changes, Italian often uses a clause with prima che + subjunctive instead.

Example:

• Aspetto prima che la torta sia pronta.

So in your sentence, prima di infornare is exactly the expected structure because the same person is doing the actions.

Italian often refers to the final item even before it is finished, especially in cooking.

So la torta can mean the cake you are making, even if at that moment it is still batter.

English does something similar:

• I’m decorating the cake
• I’m baking the cake

even though, at some stage, it is not yet fully a finished cake.

So this usage is completely natural.