der Kakao
hot chocolate
A drink usually prepared warm from cocoa powder, milk or water, and sugar.
An kalten Wintertagen trinke ich gerne einen heißen Kakao mit Sahne.
On cold winter days, I like to drink a hot chocolate with whipped cream.

cocoa
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hot chocolate
A drink usually prepared warm from cocoa powder, milk or water, and sugar.
An kalten Wintertagen trinke ich gerne einen heißen Kakao mit Sahne.
On cold winter days, I like to drink a hot chocolate with whipped cream.

Most of the time, 'Kakao' (cocoa) is an uncountable noun, especially when referring to the powder or the plant. But when you mean a cup of the drink, it can also be used countably.
Ich brauche noch Kakao für den Kuchen.
I still need cocoa powder for the cake.
You can recognize the meaning from context: verbs like 'trinken' (to drink) or words like 'Tasse' (cup) point to the drink. Quantities like 'Gramm' (grams) or baking contexts point to the powder. Terms like 'Anbau' (cultivation) or 'Ernte' (harvest) refer to the plant.
Möchtest du eine Tasse Kakao trinken?
Would you like to drink a cup of hot chocolate?
When 'Kakao' (hot chocolate) means the drink, it is often combined with 'heiß' (hot), 'kalt' (cold), 'eine Tasse' (a cup), or 'trinken' (to drink).
Ein heißer Kakao nach einem langen Spaziergang ist das Beste.
A hot chocolate after a long walk is the best thing.
In recipes, 'Kakao' (cocoa powder) almost always means pure, unsweetened cocoa powder. It is not the sweet mix that you use for a quick drink.
Achte darauf, für den Kuchen echten Back-Kakao zu verwenden.
Make sure to use real baking cocoa for the cake.
chocolate
'der Kakao' and 'die Schoko' both belong to the area of chocolate and sweet flavors. 'Kakao' can be a drink, for example warm milk with cocoa powder. It can also mean the powder used for baking or for drinks. 'Schoko' is colloquial and short for 'Schokolade' (chocolate). It refers more to the sweet food itself or to chocolate flavor. The words can be confused because chocolate is made from cocoa. In practical terms: you drink Kakao, and you eat Schoko or Schokolade.

Nach dem Spaziergang trinkt das Kind einen warmen Kakao.
After the walk, the child drinks a warm hot chocolate.
Im Kuchen ist viel Schoko, deshalb schmeckt er sehr süß.
There is a lot of chocolate in the cake, so it tastes very sweet.