Chocolate Thumbprint Cookies

0 Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat.
1 In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar together until light and fluffy, about 2-3 minutes.
2 Mix in the egg yolk and vanilla extract until well combined.
3 In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.
4 Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix.
5 Scoop out about 1 tablespoon of dough and roll into a ball. Place each dough ball on the prepared baking sheet about 2 inches apart. Use your thumb or the back of a spoon to press a small indentation in the center of each cookie.
6 Bake for 10-12 minutes, or until the cookies are set but still soft. Remove from the oven and allow them to cool slightly on the baking sheet. The cookies will spread out a bit. You can try and reshape them by swirling them inside a drinking glass.. 
7 While the cookies cool, place the chopped chocolate and heavy cream in a microwave-safe bowl. Heat in 15-second intervals, stirring in between, until the chocolate is fully melted and the mixture is smooth.
8 Spoon or pipe a small amount of the chocolate ganache into the thumbprint of each cookie while they're still slightly warm.
9 Allow the cookies to cool completely on a wire rack, letting the ganache filling set. Once cooled, dig into these rich, double chocolate treats!