Preheat the oven to 350° and line a baking sheet with parchment paper or a baking mat.
In a large bowl or the bowl of your stand mixer beat together the butter and sugars until light and fluffy, about 3 minutes.
1 cup butter,1 cup granulated sugar,½ cup brown sugar
Add the vanilla and eggs and beat to combine well.
1 teaspoon vanilla extract,2 eggs
Add the flour, cocoa, soda, and salt and stir to combine.
2 ¼ cups all-purpose King Arthur Flour,⅔ cups King Arthur's Flour double-dark Dutch cocoa,1 teaspoon baking soda,¼ teaspoon salt
Form the dough into 1 to 1 ½-inch balls. Dip the top of the ball into the heart sprinkles and then lightly press the sprinkles into place. Place on the cookie sheet.
Valentine sprinkles and/or large conversation hearts
For the conversation heart cookies, form the dough into 1 to 1 ½-inch balls and place on the cookie sheet. With the bottom of a glass of measuring cup slightly flatten the dough ball. Place a conversation heart in the center of the dough and press lightly to get it to stick into the dough.
Bake the cookies for 8 to 10 minutes until the centers are set. Let the cookies cool on the pan for 2 or 3 minutes and then place them on a wire rack.
Store extra cookies in an air-tight container.