Preheat the oven to 350°F(177C°). Line baking sheet with parchment paper or silicone baking mat. Set aside.
Put the butter in a small pan and melt over medium heat until it is halfway melted. Take it from the heat and stir until it is all melted. Set aside.
Place the flour, cocoa powder and baking soda in a small bowl and mix well. Set aside.
Place the sugar in a big bowl and add the cooled melted butter, mix well. Then add the egg and vanilla, stir until combined.
Add the flour mixture in 2 batches and mix until incorporated.
Stir in the chopped white chocolate and distribute evenly with a spatula or with your hands.
Roll the dough into 52-53 gram (2 and ½ tablespoon) balls and place them on the baking sheet. Alternatively, you can divide the dough into 12 pieces and roll each into a ball as this recipe makes 12 cookies. If you want to make smaller cookies you can roll the dough into 30 gram balls and make 21 cookies.
Bake for 8-9 minutes for the big cookies and almost 7 minutes for the smaller cookies or until the cookies just slightly set. The cookies will be too soft to handle. Remove the cookies from the oven so that they can keep their moistness. Let them cool on the baking sheet for 10 minutes or until the bottom of the cookies are firm enough. You can keep these cookies in an airtight container up to a week at room temperature and up to 3 months in the freezer. You can keep well-wrapped cookie dough in the fridge up to 3-5 days and in the freezer up to 3 months.