In a medium bowl, whisk together flour, cocoa powder, salt, and baking powder. Set aside.
In a large bowl, beat butter, brown sugar, and granulated sugar until light and fluffy, about 2 minutes.
Mix in egg yolks and vanilla until pale and creamy.
Add dry ingredients to the wet mixture and mix just until combined.
Scoop dough into 1-tablespoon portions, roll into balls, and press a thumbprint into the center. Chill for at least 1 hour.
Preheat oven to 350°F (175°C). Bake cookies for 9–11 minutes. Re-shape indents while warm if needed.
Transfer cookies to a cooling rack and allow to cool completely.
Heat heavy cream until just steaming. Pour over chocolate chips, let sit 1 minute, then stir until smooth.
Spoon ganache into cookie centers, decorate if desired, and chill 10–15 minutes until set.
