Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat.
In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar together until light and fluffy, about 2-3 minutes.
Mix in the egg yolk and vanilla extract until well combined.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix.
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.
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..
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.
Spoon or pipe a small amount of the chocolate ganache into the thumbprint of each cookie while they're still slightly warm.
Allow the cookies to cool completely on a wire rack, letting the ganache filling set. Once cooled, dig into these rich, double chocolate treats!
