Soften the butter: Place 50g of unsalted butter at room temperature to soften. It should leave an impression when pressed lightly. If in a hurry, cut the butter into small pieces and microwave at low power for 10–15 seconds, being careful not to melt it excessively.

Mix sugars with butter: Add 10g of brown sugar and 15g of granulated sugar to the softened butter. Mix evenly with a spatula until fully combined.

Prepare dry ingredients: Sift together 70g of cake flour and 3g of baking powder. Add this mixture to the butter and sugar, folding gently with a spatula. Mix lightly from the bottom up until just combined, ensuring there's no dry flour visible.

Add the egg yolk: Incorporate 1 egg yolk into the mixture and continue stirring with the spatula until the yolk is fully absorbed, making the batter smooth and consistent.

Mix in chocolate: Chop 20g of chocolate into small pieces and add it to the batter. Stir gently to distribute evenly. Preheat the oven to 180°C (356°F) at this stage if you haven't already.

Fill molds and decorate: Transfer the batter into cupcake molds, filling each to about 70–80% full. Smooth the surface with a spatula, then sprinkle the 50g of heat-resistant chocolate chips on top for decoration and added texture.

Bake the mug cookie: Place the molds into the preheated oven and bake at 180°C (356°F) for 15 minutes. Keep an eye on the cookies, and when they turn golden brown, test with a toothpick inserted into the center. If it comes out clean, they're done.