IMPORTANT: This dough must be chilled to prevent the cookies from spreading!
Prepare cookie sheets by lining with parchment paper. Set aside.
Using a stand mixer or electric hand mixer on high speed, cream sugar and butter together for 2 minutes. Add milk, egg yolk, and vanilla, then using medium speed, mix again to combine.
Add in flour and cocoa powder. Mix on low speed, just until combined. *Avoid over mixing, which can cause cookies to be dry and crumbly.
Wrap the bowl of cookie dough with plastic wrap and chill in the refrigerator for 1 hour.
When ready to bake, set oven to preheat at 350℉.
Using a small cookie scoop or tablespoon, portion dough into ½ tablespoon-sized balls. Arrange in a single layer on the prepared cookie sheet, spaced 1 to 2 inches apart. *They don't spread too much - I was able to fit 12 thumbprint cookies on a standard sized baking sheet.
Using your thumb or the back of a teaspoon, create an indent by pressing down in center of dough. *The caramel filling will be added later, after baking.
Place baking sheet on center rack of oven and bake cookies for 10-12 minutes.
Remove from oven and immediately press the center of each cookie again to make an indent, as they may have risen. Fill each indention with caramel and return to the oven for 1-2 minutes, until the caramel spreads and is smooth in the center
Optional: When cookies are cool, drizzle with melted red and green candy melts.
Notes
If you're unable to find caramel fruit dip, you can use caramel ice cream sauce or soft caramels, cut in half. Place one half of a caramel into each thumbprint indentation.
Do not skip chilling the dough. It must be cold or it will spread in the oven too much.
Storage and Freezing
Storing: Place cookies in an airtight container at keep at room temperature for up to 7 days.
Freezing baked chocolate caramel cookies is not recommended, as it causes the caramel to become grainy.
The raw cookie dough may be frozen for up to 4 months. Store in a freezer-safe container, and thaw in refrigerator before baking.