In a large bowl, mix the milk and vinegar together and set aside to curdle slightly.

Sift together the flour, cocoa powder, and baking powder in a separate bowl. Add salt and whisk to combine, then set aside.

In a double boiler over low heat, melt the dark chocolate and butter until fully liquid. Alternatively, use a microwave, stirring the mixture every 15 seconds to prevent burning.

Once melted, add the brown sugar to the milk-vinegar mixture. Using a hand blender on low speed or a whisk, mix well for about 4-5 minutes until the sugar is completely dissolved.

Slowly stream the warm chocolate-butter mixture into the milk mixture while whisking continuously. This will help emulsify the mixture.

After incorporating the chocolate, whisk for an additional 2 minutes.

Gradually fold in the flour mixture until just combined. Cover the bowl with a lid or plastic wrap and refrigerate for 10 minutes.

Once slightly cooled, use an ice cream scoop to portion out the dough (about 2 tablespoons per cookie). Roll the dough into balls, flatten slightly, and place on a baking tray. Cover the tray with plastic wrap and refrigerate for another 30 minutes.

Preheat the oven to 180°C (350°F). Remove the chilled cookie dough from the refrigerator and bake for 12-13 minutes. Adjust baking time for smaller cookies.

Allow the cookies to cool on the tray before transferring to a wire rack.