Combine the flour, cocoa powder, baking soda, and salt in a medium bowl and whisk until evenly mixed.
In a large bowl, cream the softened butter, granulated sugar, and brown sugar together until light and fluffy, about two minutes.
Add the egg and vanilla extract to the butter mixture and mix until smooth and fully combined.
Add the red food coloring and mix until the dough is evenly colored, scraping down the bowl as needed.
Pour in the buttermilk and mix on low speed until incorporated.
Add the dry ingredients to the wet mixture in two to three additions, mixing on low speed just until the flour disappears.
Divide the dough in half, shape each portion into a thick disk, wrap tightly, and refrigerate for at least one hour.
Preheat the oven to 350°F and line baking sheets with parchment paper.
Roll one chilled dough disk on a lightly floured surface to about 1/4 inch thickness.
Cut out heart shapes using a heart-shaped cookie cutter and transfer the cookies to the prepared baking sheet, spacing them about one inch apart.
Gather and reroll scraps gently, limiting rerolling to no more than two times.
Chill the cut cookies on the baking sheet for 15 minutes before baking.
Bake for 8 to 10 minutes, until the edges are set and the centers still look slightly soft.
Allow the cookies to cool on the baking sheet for five minutes, then transfer them to a wire rack to cool completely.