Cream together the softened butter and granulated sugar in a large mixing bowl until light and fluffy.

Add the egg and vanilla extract, beating until well combined.

Whisk together the flour, cocoa powder, baking powder, and salt in a separate bowl.

Gradually mix the dry ingredients into the creamed mixture at low speed until just incorporated.

Divide the dough in half, shape into discs, wrap in plastic wrap, and refrigerate for about 1 hour.

Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

Roll out one disc of dough to about 1/8 to 1/4 inch thickness. Cut out cookies using a heart-shaped cookie cutter and transfer to the baking sheet.

Bake for 9-10 minutes, then let cool completely on a wire rack.

Beat together the softened butter, half and half, vanilla, and salt for the filling.

Gradually add the powdered sugar until the mixture is smooth and slightly stiffer than cupcake frosting.

Transfer the filling into a piping bag, pipe onto half of the cookies, and top with the remaining cookies to create sandwiches.