Preheat your oven to 350°F (175°C). Line your baking sheets with parchment paper.

In a large bowl, beat the softened butter and sugar together until the mixture is light and fluffy.

Mix in the egg and vanilla extract until well combined.

In another bowl, whisk together the flour, baking powder, salt, and cocoa powder until blended.

Gradually add the dry mixture to the wet mixture. Mix until it forms a dough.

Divide the dough into three equal parts. Leave one part plain (vanilla), mix the cocoa powder into the second part (for chocolate), and add the jam to the third part (for strawberry).

Take small portions from each flavor, roll them into balls, and layer them together. Press them slightly to shape.

Place the shaped cookies onto the prepared baking sheets and bake for 10 to 12 minutes or until the edges turn golden.

Let the cookies rest on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.