Preheat your oven to 170°C (340°F) with top and bottom heat. Let butter soften at room temperature until it's easily pressed but not melted. Bring your eggs to room temperature. Melt the dark chocolate over a double boiler and let it cool slightly before using.

In a large bowl, combine softened butter and powdered sugar. Mix with a spatula until partially combined.

Add the beaten egg in two additions, mixing well after each one to prevent curdling.

Sift in the cake flour and fold until no dry flour remains. Gently knead into a smooth dough — it should feel soft but not sticky.

Wrap in plastic wrap and refrigerate for 30 minutes to firm up before shaping.

In another bowl, mix softened butter and granulated sugar until smooth.

Add cooled melted chocolate and the egg, whisking until fully combined and silky.

Sift together cake flour, cocoa powder, and baking powder, then fold into the mixture until no dry flour remains. Gently knead into a smooth dough.

Wrap and chill for 30 minutes to relax and firm up for shaping.

Take both doughs from the fridge. Divide the cookie dough into 12 g portions and the brownie dough into 15 g portions, rolling each into a ball.

Flatten one cookie dough ball slightly in your palm. Place a brownie dough ball in the center and wrap the cookie dough around it completely, pinching to seal the seam.

Roll into a smooth ball again, place on a baking tray, and gently press to about 1.5 cm thick, leaving 5 cm between each cookie.

Bake in the preheated oven (middle rack) at 170°C (340°F) for 15 minutes.

The cookies should look set with lightly golden edges — don't overbake or the brownie center will dry out.

Cool completely on a wire rack for about 20 minutes; as they cool, the outer cookie becomes delightfully crisp while the inside stays soft and fudgy.