1. Prepare the Crust:

1. Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.
2. In a large bowl, cream together the softened butter and sugar until light and fluffy. Beat in the egg yolks and vanilla extract.
3. In a separate bowl, whisk together the flour, cocoa powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until a soft dough forms.
4. Roll the dough into 1-inch balls and place them 2 inches apart on the prepared baking sheets. Use your thumb or the back of a teaspoon to make an indentation in the center of each cookie.
5. Bake for 10-12 minutes, or until the cookies are set. If the indentations have puffed up, gently press them down again while the cookies are still warm. Let the cookies cool completely on the baking sheets.
6. For the ganache, place the chopped chocolate in a heatproof bowl. Heat the heavy cream until just simmering, then pour it over the chocolate. Let it sit for 2 minutes, then whisk until smooth. Spoon the ganache into the indentations of the cooled cookies and top with sprinkles immediately.