In a large bowl, beat the softened butter with a mixer on medium for about 30 seconds.

Add the sugar and beat until combined, scraping the sides of the bowl as needed.

Beat in the egg yolks and vanilla until combined.

Beat in the all-purpose flour and unsweetened cocoa powder.

Cover the dough and chill for 1 hour or until it's easy to handle.

Preheat your oven to 375°F.

Grease a cookie sheet.

Shape the dough into 1-inch balls.

If desired, roll the balls in egg whites, then in chocolate-flavor sprinkles to coat.

Place the balls 1 inch apart on the prepared cookie sheet.

Using your thumb, make an indentation in the center of each ball.

Bake for 10 to 12 minutes or until the bottoms are light brown.

If the cookie centers puff during baking, re-press the centers with the rounded side of a measuring teaspoon.

Remove the cookies from the oven and immediately press a caramel-filled dark or milk chocolate into each center.