In a large bowl, cream together 1 cup softened butter and 1 cup sugar until light and fluffy.

Beat in the egg and vanilla extract until well combined.

Whisk together flour, cocoa powder, and salt, then gradually add to the wet ingredients until a dough forms.

Shape the dough into 1-inch balls and place on a parchment-lined baking sheet.

Press a thumb or small measuring spoon into the center of each ball to create an indentation.

Bake at 350°F (175°C) for 10 minutes or until set.

Heat heavy cream in a small saucepan until it reaches a simmer, then pour over the chopped dark chocolate and 1 tablespoon butter.

Let the ganache sit for 5 minutes, then stir until smooth.

Spoon the warm ganache into the center of each cooled cookie.

Top with heart-shaped sprinkles and allow the ganache to set at room temperature.