Вечером мы решили испечь новый корж для торта, потому что первый корж получился слишком тёмным.

In Russian, some time expressions are commonly used in the instrumental case without a preposition.

So:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

Here, вечером simply means in the evening. It is a fixed and very common time expression.

Решили is the past tense plural form of решить (to decide).

The subject is мы (we), so the verb must also be plural:

• я решил / решила = I decided
• ты решил / решила = you decided
• мы решили = we decided

So мы решили means we decided.

Also, решить is a perfective verb, which fits well here because it refers to a completed decision.

This is a question of aspect.

• печь = imperfective: to bake, to be baking, to bake in general
• испечь = perfective: to bake completely, to bake successfully as one finished action

After решили in this sentence, испечь is used because the idea is to bake one new cake layer as a completed action.

So:

• решили печь could suggest the process or a more general plan
• решили испечь means decided to bake (and finish baking) one layer

That is the most natural choice here.

Корж is a cake layer or sponge layer used to make a cake.

A торт is the whole cake, while a корж is one of the baked layers inside it.

So:

• торт = cake
• корж = cake layer

In this sentence, they are baking a new layer for the cake, not a whole new cake.

Because the preposition для (for) requires the genitive case.

The base form is:

• торт = cake

In the genitive singular, it becomes:

• торта

So:

• для торта = for the cake

This is a very important pattern:

• для мамы = for mom
• для друга = for a friend
• для торта = for the cake

Russian often repeats nouns where English might prefer it or the first one.

So instead of saying something like:

• ...because the first one turned out too dark

Russian naturally says:

• ...потому что первый корж получился слишком тёмным

This repetition is completely normal and often makes the sentence clearer.

A pronoun could be possible in some contexts, but repeating корж sounds very natural here.

Here получился means something like:

• turned out
• came out
• ended up being

So:

• первый корж получился слишком тёмным = the first cake layer turned out too dark

This is different from был (was).

• был слишком тёмным simply describes it as being too dark
• получился слишком тёмным emphasizes the result of the baking process

This is very common with food, creative work, plans, and results:

• Суп получился вкусным. = The soup turned out tasty.
• Торт получился красивым. = The cake turned out beautiful.
• Экзамен получился трудным. = The exam turned out difficult.

Because after получился in this kind of meaning (turned out to be), Russian commonly uses the instrumental case for the describing word.

So:

• тёмный = nominative masculine singular
• тёмным = instrumental masculine/neuter singular

Since корж is masculine singular, the adjective becomes тёмным.

This pattern is very common after verbs like:

• быть (in some contexts)
• стать
• оказаться
• получиться

Examples:

• Он стал врачом. = He became a doctor.
• Она оказалась правой. = She turned out to be right.
• Суп получился солёным. = The soup turned out salty.

So слишком тёмным means too dark in the grammatical form required here.

Слишком means too.

So:

• слишком тёмным = too dark

It comes before the adjective or adverb it modifies, just like English too:

• слишком большой = too big
• слишком медленно = too slowly
• слишком тёмным = too dark

Because потому что introduces a subordinate clause meaning because.

The sentence has two parts:

1. Вечером мы решили испечь новый корж для торта
2. потому что первый корж получился слишком тёмным

In Russian, subordinate clauses are normally separated by a comma, so the comma before потому что is required.

Yes, the word order is natural and slightly emphasizes the time setting first.

• Вечером мы решили... = In the evening, we decided...

Russian word order is more flexible than English, and placing вечером first helps set the scene immediately.

Other orders are possible, for example:

• Мы решили вечером испечь новый корж...

But that can sound a little more like we decided to bake it in the evening, while Вечером мы решили... more naturally means In the evening, we made the decision.

So the original order is very natural.

Here новый means a new/replacement one, not necessarily brand-new in some abstract sense.

So новый корж means:

• a new cake layer
• another cake layer to replace the bad one

This is exactly what English would usually mean by a new layer in this context.

Yes, that is a good close translation.

A more natural English version might be:

• In the evening, we decided to bake a new cake layer because the first one had turned out too dark.

But grammatically, your more literal version matches the Russian structure quite well.