Start by cracking 4 eggs in your stand mixer bowl, add 2/3 cup (140 g) brown sugar and 3 tbsp (40 g) granulated sugar. Use the whisk attachment and beat on high speed for 10-15 minutes. They will turn very pale and thick.
Melt slightly over 1 cup (250 g) of unsalted butter. Once fully melted, turn off the stove and add 1 ¾ cups (300 g) of chopped milk chocolate couverture. Let it sit untouched for 5 minutes. After 5 minutes, stir until the chocolate is fully melted.
In a mixing bowl, add ¼ tsp salt, ¾ cup (90 g) all-purpose flour, and 1 cup (90 g) cocoa powder. Whisk until combined. You can sift it to remove any lumps (recommended).
Pour the melted chocolate mixture over the dry ingredients. Mix to incorporate. Then, fold in the pale beaten eggs until just incorporated. Bake in a preheated oven for 20-25 minutes, depending on how you want the center to be.