For the brownie base:

1/2 cup unsalted butter, melted

1 cup granulated sugar

2 large eggs

1 tsp vanilla extract

1/3 cup unsweetened cocoa powder

1/2 cup all-purpose flour

1/4 tsp salt

1/4 tsp baking powder

For the chocolate mousse:

1 cup heavy whipping cream, chilled

4 oz semisweet chocolate, finely chopped

2 tbsp milk

2 tbsp powdered sugar

For the ganache topping:

4 oz semisweet chocolate, finely chopped

1/2 cup heavy cream

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Make the brownie base: Preheat oven to 350 °F (175 °C). Line an 8×8-inch pan with parchment. In a bowl, whisk melted butter and sugar. Stir in eggs and vanilla. Sift in cocoa powder, flour, salt, and baking powder; fold gently. Pour into pan and bake for 20–25 minutes until center is just set. Cool completely.
Prepare the mousse: Melt chopped chocolate with milk in a double boiler or microwave until smooth. Cool to room temp. Whip chilled cream and powdered sugar to stiff peaks. Gently fold whipped cream into cooled chocolate. Spread mousse evenly over brownie base. Chill for 1 hour.
Finish with ganache: Heat heavy cream and pour over chopped chocolate. Let sit 1–2 minutes, then stir until glossy and smooth. Cool slightly and pour over mousse. Smooth top and refrigerate for 30 minutes until set.
Serve: Use parchment overhang to lift brownies out. Slice into squares with a hot, sharp knife for clean edges.

Use parchment paper for easy removal and neat slices.

Let each layer cool/set before adding the next for clean definition.

Add espresso powder or swirl nut butter into brownie layer for variations.

Top ganache with nuts or fresh berries if desired.

Chill before slicing and wipe knife between cuts for best presentation.