Heat oven to 170°C/340°F on convection. Line a 20×20 cm/8×8-inch square pan with parchment paper.
Melt butter and dark chocolate together until smooth. Stir until fully combined.
Whisk in eggs, sugar, and salt just until smooth. Fold in flour and baking powder.
Pour batter into the prepared pan and smooth the surface. Bake for 25 to 30 minutes, until the edges are set and the center jiggles slightly.
Cool completely, then freeze uncovered until fully firm, at least 4 hours or overnight.
Melt milk chocolate gently without oil. Stir in ground almonds, if using.
Cut brownies into bite-size squares. Dip each piece into the chocolate, letting excess drip off.
Place on parchment paper and freeze until the coating is set, about 1 hour. Serve chilled or at room temperature.