Melt the butter in the microwave or in a small saucepan over low-medium heat. You don’t want it to bubble, so as to not lose any of the liquid in the butter. Pour the melted butter into a big mixing bowl and let it cool down to room temperature in the fridge. Takes about 20 minutes.
150 g butter
Once the butter has reached room temperature add the granulated sugar and brown sugar and with a rubber spatula whisk it together for 1 minute. Alternatively, mix it together in a stand mixer, on medium speed, using the paddle attachment.
200 g granulated sugar, 50 g dark brown sugar
Add the egg, egg yolk and vanilla extract and mix it in until combined.
1 egg, 1 teaspoon vanilla extract, 1 egg yolk
In a separate bowl, stir together flour, cocoa powder, baking powder, baking soda and salt. Add the dry ingredients and with spatula mix it together until just combined.
260 g all-purpose flour, 40 g cocoa powder, ¾ teaspoon baking powder, ¾ teaspoon baking soda, ¾ teaspoon salt
Place the cookie dough in the fridge to set for 30 minutes. Meanwhile, preheat the oven at 175ºC / 350ºF and prepare a baking sheet with baking paper.
Using a medium 2 ounce cookie scoop / ¼ cup (55 g pr cookie), scoop the dough and roll it between your hands into a ball. Place it on a baking tray lined with baking paper and gently press the cookie dough ball to flatten the top. They should be about 2 cm/0,8 inch thick.
Place 6 cookies per baking tray and bake one tray at a time for 9-10 minutes. Place the cookie dough back in the fridge while the cookies bake. It will yield 14 cookies in total.
Once baked, let them cool down on the baking tray for 3 minutes as they still will be soft when they are done. After they have cooled down a bit, use a spatula to lift them onto a cooling rack and let them cool down completely.