Preheat oven to 350°F (177°C) degrees. Spray with nonstick cooking spray.
In a stand-up mixer using a paddle attachment, on medium speed, cream together butter (1/2 cup) and sugar (2/3 cup), until light and fluffy, about 2 minutes.
Add the egg yolk (1), milk (2 tablespoons). and vanilla (1 teaspoon) and beat until combined.
In a separate mixing bowl, whisk together flour (1 cup), cocoa (1/3 cup), and salt (1/4 teaspoon).
Gradually add the flour mixture to the butter mixture and mix well.
Shape dough using a medium cookie scoop then roll into a ball.
Place balls 2 inches apart on prepared baking sheets. Using 1/2-teaspoon measure, make an indentation in center of each ball. Refrigerate for 2-30 minutes.
Bake until set, about 10-12 minutes, switching and rotating sheets halfway through baking.
Make The Chocolate Ganache Filling:
Place the chocolate chips (1 cup) in a mixing small bowl.
Heat the heavy whipping cream (1/2 cup) in the microwave, or on the stove, until just about boiling.
Pour the heavy cream over the chocolate chips and let sit for 1 minute.
Briskly whisk together the cream and chocolate until smooth.
Assemble:
Fill each well of the cookies with ganache.
Chill the cookies in the fridge for 10-15 minutes to allow the ganache to set.
If desired, garnish with jimmies, sprinkles, or nonpareils.
Notes
Measuring Dough: I use a medium cookie scoop to measure out the dough with the dough leveled off.
Shaping: Roll the scooped dough into balls and use a 1/2-inch rounded measuring spoon to make the well. I also use the end of a wooden spoon to help shape the well. I like to make it as big as possible. The dough is very soft at this point so the shape of the cookies may not be as sharp as you like. See below.
Refrigerate The shaped cookies for 2-30 minutes to firm up the butter. Take out and refine the shape of the cookies as desired.
After Baking: Cookies’ shape can also be refined by re-pressing after baking and they’re warm.