Las natillas están muy buenas, y en esa panadería también venden empanada de atún.

In Spanish, natillas is normally used in the plural, even when English would often say custard as an uncountable singular noun.

So:

• Las natillas están muy buenas = The custard is very good

This is just how the word is commonly used. Spanish often treats some foods or dishes differently from English in terms of singular/plural.

It uses están because here estar is used to describe a state or condition, especially how food tastes or seems right now.

• Las natillas están muy buenas = The custard tastes very good / is really good

With food, Spanish very often uses estar bueno/a/os/as to mean to taste good or to be delicious.

Using ser here would sound unnatural in this context. Ser bueno usually refers more to general qualities, not the immediate experience of tasting the food.

Because buenas has to agree with natillas.

• natillas is feminine plural
• so the adjective must also be feminine plural: buenas

Agreement pattern:

• el pastel está bueno
• la empanada está buena
• los pasteles están buenos
• las natillas están buenas

This kind of adjective agreement is a basic feature of Spanish grammar.

Because bueno/a/os/as is an adjective, and it describes the noun natillas.

• Las natillas están muy buenas = The custard is very good / tastes great

Bien is usually an adverb, so it is used differently:

• Cocina bien = He/She cooks well
• Estoy bien = I am well / I’m fine

With food, estar bueno/a is the normal pattern, not estar bien if you mean delicious.

Esa means that, referring to something not right next to the speaker.

Spanish has a three-way distinction:

• esta = this (near the speaker)
• esa = that (near the listener or at some distance)
• aquella = that over there (far from both)

So:

• en esa panadería = in that bakery

In many everyday situations, English only uses this/that, but Spanish often makes a finer distinction.

Because en is the normal preposition for saying that something happens in or at a place.

• en esa panadería = in/at that bakery

With shops and businesses, Spanish often uses en where English might say either in or at.

Examples:

• Trabaja en una farmacia = She works at/in a pharmacy
• Compré pan en esa panadería = I bought bread at that bakery

También usually goes before the verb or before the part of the sentence it modifies.

Here:

• también venden empanada de atún = they also sell tuna empanada

This is a very natural word order in Spanish.

You could sometimes move también for emphasis, but this version is the most straightforward and common:

• En esa panadería también venden empanada de atún

In Spanish, the subject is often left out because the verb ending already gives information.

• venden = they sell

Here, they does not necessarily mean specific people already mentioned. It often means:

• they in a general sense
• people there
• the bakery / the staff

So también venden is a natural way to say:

• they also sell
• they also have for sale

Spanish does this very often with verbs when the subject is understood from context.

Yes, Spanish has more than one natural way to express this idea.

• También venden empanada de atún = They also sell tuna empanada
• También se vende empanada de atún = Tuna empanada is also sold / They also sell tuna empanada

The version with venden sounds more direct and conversational. It focuses on the people or business doing the selling, even if the subject is not stated.

The se version is more impersonal. Both are possible, but venden is very common in everyday speech.

Because in Spanish, when talking about what a shop sells, it is common to mention a product without an article, especially when referring to it as a type of item rather than one specific unit.

So:

• venden empanada de atún = they sell tuna empanada

This is similar to saying:

• Venden pan
• Venden café
• Venden tortilla

If you said una empanada de atún, it could sound more like a tuna empanada as one item. Without the article, it sounds more like the bakery carries that product generally.

De atún means the empanada is a tuna empanada, meaning tuna is the main filling or defining ingredient.

• empanada de atún = tuna empanada

Con atún would mean with tuna, which is possible in some contexts, but it is less natural for naming the type of food.

Compare:

• bocadillo de jamón = ham sandwich
• pizza de queso = cheese pizza
• empanada de atún = tuna empanada

So de is often used to classify or name foods by their main ingredient.

Yes. Panadería is a bakery, a shop where bread and often pastries or other baked goods are sold.

In Spain, a panadería may also sell some sweet or savory items besides bread, so it is not strange that they sell empanada de atún there.

That said, depending on the place, products can vary. The sentence is completely natural.

Yes. In Spain, natillas are a very familiar traditional dessert, similar to custard. So this sentence sounds culturally natural for Spain Spanish.

A learner may notice that food words are often very culture-specific, and natillas is one of those words that is especially common in Spain.