Tout simplement parce qu’elle fera plusieurs crêpes avec sa recette.

Crêpes est ici le résultat de la recette, en réalité elle fabrique une pâte à crêpe pour avoir des crêpes.

You don’t translate languages word for word. Each language obeys to its own grammatical and lexical rules, and we translate ideas not words.

In the English sentences:

some chocolate crepe recipes
a chocolate crepe recipe

crepe is an adjective that modifies “recipe”/”recipes”, and in English adjectives do not agree with the noun.

In the French sentences:

des recettes de crêpes au chocolat
une recette de crêpes au chocolat

crêpes is a noun and is plural if one considers there are multiple items represented. It is usual to make several crepes with one batch of dough. But of course if your sister has the technology for it and usually makes one huge chocolate crepe that you will split into portions as you would for a chocolate cake than you could write:

Ma sœur a des recettes de crêpe au chocolat

implying the recipe will probably include the technical details for making one huge crepe in a single go.

Attention aussi à certains pièges,

Une recette de bœuf bourguignon

On considère qu’on utilise du bœuf (viande), et non pas des bœufs (animaux).