Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. Add the vanilla extract and egg, mixing until well combined.
In another bowl, sift together the flour, cocoa powder, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, mixing until a dough forms.
Roll the dough into small balls and place them on the prepared baking sheet. Flatten each ball slightly with your hand.
Bake for 8 to 10 minutes, until the cookies are set. Allow them to cool completely on a wire rack.
To make the filling, beat the heavy cream, vanilla extract, and powdered sugar together until smooth and creamy.
Once the cookies have cooled, spread the filling on the flat side of one cookie and sandwich it with another cookie.