В булочной мы взяли по одному круассану и по чашке кофе.

Because в meaning in/at requires the prepositional case when it indicates location.

• булочная = bakery
• в булочной = in/at the bakery

So the noun changes from nominative булочная to prepositional булочной.

This is a very common pattern:

• в школе = at school
• в комнате = in the room
• в булочной = at the bakery

Also, булочная is one of those Russian nouns that historically comes from an adjective, so some of its case forms may look adjective-like.

Булочная is a shop connected with bread and baked goods, often translated as bakery or bread shop.

Depending on context, it can suggest a place where you buy things like:

• bread
• rolls
• pastries
• croissants

So in this sentence, в булочной simply means at the bakery.

Взяли is the past tense plural of взять, which is a perfective verb meaning to take, to get, or in this kind of context often to have / buy / pick up.

Why plural? Because the subject is мы = we.

So:

• я взял / я взяла = I took
• мы взяли = we took/got

Why perfective? Because the action is seen as a completed whole: they went to the bakery and got the items.

In food-buying contexts, взять can sound very natural in Russian where English might say:

• we got
• we had
• we bought
• we took

Here по has a distributive meaning: one each, apiece, for each person.

So:

• по одному круассану = one croissant each
• по чашке кофе = a cup of coffee each

This is a very common Russian structure when each person receives the same amount of something.

Compare:

• Мы взяли два круассана. = We took two croissants.
  (just the total number)

• Мы взяли по одному круассану. = We took one croissant each.
  (distribution per person)

Because after по in this distributive meaning, Russian normally uses the dative case.

So both words are in forms required by that pattern:

• один → одному
• круассан → круассану

That gives:

• по одному круассану = one croissant each

A learner-friendly way to think of it is:

• по + dative = each / apiece

This pattern is extremely common:

• по книге = one book each
• по билету = one ticket each
• по одному яблоку / по одному яблоку each — though number patterns can vary, the key idea is still distribution

Both are possible, but they are slightly different in style.

• по чашке кофе = a cup of coffee each
• по одной чашке кофе = one cup of coffee each

The version without одной is often more natural when the idea of one each is already clear from по.

So in everyday Russian, по чашке кофе is very normal and idiomatic.

If you add одной, you emphasize the number one a bit more.

Again, because of по.

The noun after distributive по usually goes into the dative singular here:

• чашка → чашке

So:

• по чашке кофе = a cup of coffee each

This is the same pattern as:

• по ложке сахара = a spoonful of sugar each
• по тарелке супа = a bowl of soup each

There are two useful things to know here.

1. Кофе is often treated as an indeclinable noun in form, so it frequently looks the same in different cases.
2. In чашка кофе (a cup of coffee), the second noun is part of a measure/container expression, where English also uses of:
   • a cup of coffee
   • чашка кофе

So even though кофе is functioning like coffee after cup of..., its form stays кофе.

For a learner, the simplest takeaway is:

• memorize чашка кофе
• then in this sentence: по чашке кофе

Because Russian normally repeats по with each distributed item.

So:

• по одному круассану и по чашке кофе
  = one croissant each and a cup of coffee each

Repeating по makes the structure clear and natural. It tells you that both parts are distributive.

If you removed the second по, the sentence would sound less standard and could become less clear.

Yes. In this context, взять is broader than literal physical taking.

At a bakery or café, мы взяли... can mean:

• we got...
• we ordered...
• we bought...
• we had...

Russian often uses взять in these everyday consumer situations where English chooses different verbs depending on context.

So although the basic meaning of взять is to take, the natural English translation often depends on the situation.

It is talking about what each person got.

That is exactly what по signals here:

• по одному круассану = one croissant each
• по чашке кофе = one cup of coffee each

So if мы means two people, the total would be:

• two croissants
• two cups of coffee

If мы means three people, then the total would be:

• three croissants
• three cups of coffee

Russian does not state the total here; it states the distribution per person.