In a large mixing bowl, combine butter and sugar. Cream until light and fluffy.

Beat in egg yolk, milk, and vanilla.

In a large bowl, whisk together flour, cocoa, and salt.

Add dry ingredients to the wet ingredients in the mixing bowl, and beat until just combined. If batter is crumbly, add about 1 tbsp. milk, or enough that dough easily forms into balls.

Shape into 1-inch balls.

Roll each ball in red sprinkles until well-coated.

Place two inches apart on parchment paper or silicone mat-lined baking sheets.

Press an indentation in the center of each dough ball using your thumb or the end of a wooden spoon.

Bake at 350 degrees for 10-12 minutes, or until set when you lightly touch an edge (carefully).

If desired, you can use the end of a wooden spoon to reshape the indentation in the cookies.

Remove cookies to wire racks to cool completely.

In a small microwave-safe bowl, heat strawberry preserves for 30 seconds, or until thin and runny.

Spoon about 1/2 tsp. into each thumbprint. You want it to fill the well of each thumbprint about 2/3-full.

Place in the fridge for 10 minutes, or until jam is set.

In a small microwave-safe bowl, combine chocolate chips, heavy cream, and butter.

Microwave on high at 30-second intervals until chocolate is melted and the mixture is smooth.

Pipe or spoon into the center of each thumbprint, covering the strawberry jam.

Allow 10-15 minutes for the ganache to set.

Drizzle with melted chocolate chips, if desired. Allow to set, and devour!