01 -
Set your oven to 350°F (175°C) first. Lay parchment in your 8×8-in (20×20 cm) pan. Start by whisking butter, eggs, vanilla, and sugar together until it looks nice and smooth. Drop in the dry stuff—cocoa, baking powder, flour, and salt—after you sift them. Fold that in, but don't fuss over mixing too much. Move all the batter into your lined pan and even it out.
02 -
Pop the pan in the oven and bake for about 20 to 25 minutes. Stick a toothpick in the center—it's ready if you just see some sticky crumbs. Let it completely cool off before you touch it.
03 -
Chill your mixing bowl in the fridge if you want. Get that milk hot (not bubbling) on the stove, then pour it over the chopped chocolate in a bowl. Wait a couple minutes. Mix until it's silky and glossy, then let it sit until it's room temp but still easy to stir. In another bowl, beat the heavy cream with the powdered sugar till you see soft fluffy peaks. Fold the chocolate in gently, trying to keep it airy. Spread the mousse all over the cooled brownies. Stash it in the fridge for at least a couple hours so it sets up.
04 -
Put that chopped chocolate in a bowl. Heat your cream in a saucepan till it's steaming hot—don't let it boil. Pour it on top of the chocolate. Give it 2 or 3 minutes, then stir till you get a glossy ganache. Pour this over your set mousse and spread it out smoothly. Pop the pan back in the fridge until the chocolate is firm—about 1 hour.
05 -
Once the top is set totally firm, grab the edges of your parchment and lift everything out of the pan. Grab a sharp knife and cut into 12 squares. Wipe off the blade between each cut if you want those layers looking sharp.