In a medium bowl, whisk together flour, cocoa powder, salt and baking soda. In a large bowl, using a mixer, beat softened butter with granulated sugar until fluffy.
Add in egg and vanilla extract, beat until smooth. Add the dry ingredients into the mixture in three additions, beating well after each until all of the flour is incorporated. Cover and refrigerate dough for at least one hour.
Preheat oven to 350 F. Divide dough into smaller balls and roll out each onto a floured surface. Roll out into a rectangle and cut out cookies using a heart shaped cookie cutter. Place cookies onto a large ungreased baking sheet and bake for 10-12 minutes.
