1-Step 1: Prepare the pan and oven. Preheat your oven to 350°F. Line a 7 x 11 inch pan with foil or baking paper. This makes it easier to lift the brownie layer out later, which matters when you need to cut neat little cake rounds.

2-Step 2: Make the brownie base. Place 1/2 cup unsalted butter and 7 oz semi-sweet chocolate in a saucepan over low heat. Stir gently until melted and smooth. Once the mixture is combined, stir in 3/4 cup granulated sugar. Let it cool slightly before adding the eggs so they do not scramble. Add 2 large eggs and 1 teaspoon vanilla, then mix until glossy. Fold in 1/2 cup plus 2 tablespoons flour, 2 tablespoons unsweetened cocoa powder, and 1/4 teaspoon salt. Stir just until combined. A few streaks of flour are better than overmixing, because overmixing can make the brownies tough.

3-Step 3: Bake and cool the brownies. Pour the batter into the prepared pan and spread it evenly. Bake for 20 to 25 minutes at 350°F. The brownies should look set, with a slightly soft center that will finish firming as they cool. Let the pan cool fully before handling the base, because warm brownies can tear when you cut them.

4-Step 4: Cut the brownie disks. Once the brownies are cool, cut them into 6 disks. If your cutter is sticking, wipe it clean between cuts. Then make baking paper collars for each brownie disk. These collars act like little molds and help the mousse layers stay neat and tall. Use the baking paper collars like tiny cake rings. They are simple, cheap, and they make the finished cakes look much more polished.

5-Step 5: Whip the cream for the mousse. Whip 1 2/3 cups chilled heavy whipping cream until soft peaks form. Do not overwhip it, or the mousse can become grainy. Keep the whipped cream cold while you work with the chocolate mixtures.

6-Step 6: Make the dark chocolate mousse layer. For the first layer, sprinkle 3/4 teaspoon unflavored gelatin powder over 1 tablespoon cool water. Let it sit briefly to bloom. Add 1/4 cup heavy cream and heat gently until the gelatin dissolves. Stir in 5.5 oz chopped small dark chocolate until melted and smooth. Let the mixture cool slightly, then fold it into a portion of the whipped cream. Pipe or spoon this mousse over the brownie disks inside the paper collars. Smooth the tops lightly.

7-Step 7: Make the milk chocolate mousse layer. Repeat the same process with another 3/4 teaspoon unflavored gelatin powder, 1 tablespoon cool water, 1/4 cup heavy cream, and 5.5 oz chopped small milk chocolate. Fold into the whipped cream and pipe carefully over the dark chocolate layer. Chill briefly if needed so the layers stay distinct.

8-Step 8: Make the white chocolate mousse layer. Repeat once more with the final 3/4 teaspoon unflavored gelatin powder, 1 tablespoon cool water, 1/4 cup heavy cream, and 5.5 oz chopped small white chocolate. Fold into the whipped cream and add the final mousse layer on top. This gives the cakes their signature triple chocolate look.

9-Step 9: Chill until set. Place the cakes in the refrigerator for 4 to 5 hours. This chilling time is important because it helps each layer set properly and gives you clean slices later. If you are making these for guests, you can chill them overnight for even firmer results.

10-Step 10: Finish with ganache and curls. To make the ganache, heat 1/4 cup heavy cream until steaming, then pour it over 4 oz chopped milk chocolate. Let it sit for a moment, then stir until smooth. Remove the collars from the cakes, spoon or drizzle the ganache on top, and finish with semisweet chocolate curls made from the block of semisweet baking chocolate. For a fruitier dessert table, these cakes pair nicely with blueberry cream cake if you want a colorful mix of flavors and textures.