In a small bowl, add in the peanut butter, powdered sugar, and vanilla extract. Stir with a spatula until smooth. Set aside.
Preheat the oven to 350 degrees F. Line a cookie sheet with parchment paper.
In a medium bowl, whisk together flour, cocoa powder, and salt. Set aside.
In a large mixing bowl, add in the butter and sugar. Beat until light and creamy.
Add in the egg yolk and vanilla extract. Beat until incorporated.
Add the dry ingredients and beat until the dough comes together.
Using a medium cookie scoop, scoop the dough into balls and drop them 2 inches apart on the
prepared cookie sheet. Flatten the top of the dough balls, but don't squish them like pancakes. You just want the top and bottom to flatten out slightly and the center still very thick.
Using the back of a ½ teaspoon measuring spoon, make an indentation in the center of each dough ball. You can also use your finger.
Scoop about 1 teaspoon of the peanut butter filling and roll it between your palms. Pop it into the whole that you formed. Be sure to fill your thumbprint completely because the cookie won't puff up and cover up any of the edges. Repeat with all cookies.
Place into the oven and bake for 15 minutes or until the edges of the cookie are firm.
Remove from the oven and let sit on the baking sheet for 5 minutes then transfer to the cooling rack.
Once the cookies are cooled completely, melt the baking chocolate for the drizzle. Do this by adding the chopped baking chocolate into a microwave-safe bowl. Microwave for 30 seconds and stir. Repeat microwaving for 30 second intervals and stirring until the chocolate is smooth.
Add the chocolate to a sandwich bag and cut a small corner out of the bag. Drizzle the chocolate on the cookies by quickly moving your hands back and form while applying even pressure to the sandwich bag with the melted chocolate. Let the chocolate cool down and harden.