In a mixing bowl, combine the flour, cocoa powder, baking powder, and salt. Set aside.
Place the butter and chocolate in a microwave-safe bowl and heat 10 seconds at a time, mixing in between, until the chocolate and butter have melted and combined. Set aside.
In a stand mixer fitted with a whisk attachment (or use a mixing bowl and a hand mixer), whisk the egg and sugar for 10 minutes on medium-high until tripled in size and pale.
Then, slowly add the chocolate mixture while whisking on medium until incorporated.
Add the dry ingredients and mix with a rubber spatula or use a paddle attachment until no dry ingredients are left. Then, add solid chocolate chips and mix to incorporate.
When done, cover the dough with plastic wrap and refrigerate in a bowl for 20 minutes.
Meanwhile, preheat the oven to 180C (355F) and prepare the baking sheet by lining it with parchment paper.
Take the dough out of the fridge and make 10 even balls (use a cookie scoop or your hands) and place them on the tray, leaving enough space in between as cookies will spread during baking. You can bake them in two batches if you don’t have enough space on the sheet – just leave the second batch in the fridge while the first batch is baking.
Bake for 12-13 minutes or until the cookies have spread and become crackly on the top. They might look underbanked, but they are not. Take the sheet out and let the cookies cool down on it for 10-15 minutes. Then, move them onto a cooling rack to cool down completely.
Repeat the baking process with the second batch, if you have it.