In a large bowl, whisk together flour, cocoa powder, baking powder, and salt.
In the bowl of a stand mixer fitted with the paddle attachment, cream together butter and sugar for 2 minutes until light and fluffy. Then add in the egg and vanilla extract and beat for 2 more minutes.
Slowly add in the flour mixture and mix until all combined and well-incorporated.
Divide dough in half and shape into two discs. Wrap each disc in plastic wrap and chill in the fridge for 30 minutes.
In the meantime, preheat oven to 350 °F (177 °C) and line baking sheets with parchment paper or silicone baking mats.
Taking a portion of the dough out of the fridge at once, turn dough onto lightly floured surface and roll the dough to about 1/4-inch thickness. Use a heart-shaped cookie cutter (or a round cookie cutter) to cut out the dough and transfer them to the prepared baking sheet.
Repeat until all your dough has been used up. Bake for 12 minutes then transfer to wire rack to cool completely.
While the cookies are cooling, make your buttercream.
Cream together butter and half and half, then slowly add the powdered sugar. Once it's all combined, about 2 minutes, add the red food coloring. Add more for a darker red color, if desired.
Pipe the buttercream onto one half of the cooled cookies and sandwich together with another half.