Preheat the oven to 170°C / 340°F with top and bottom heat.

Line two 20 x 20 cm square cake pans with baking paper.

Place 100 ml whole milk, 120 g unsalted butter, and 110 g dark cooking chocolate in a heatproof bowl.

Set the bowl over gently simmering water and stir until the mixture is melted and smooth.

Remove the chocolate mixture from the heat and let it cool to room temperature.

Sift 110 g wheat flour, 20 g cocoa powder, 2 g baking powder, 2 g baking soda, and salt into a mixing bowl.

Beat 3 eggs and 120 g granulated sugar until pale, thick, and expanded.

Fold the sifted dry ingredients into the egg mixture until mostly combined.

Add the cooled chocolate mixture and butter flavoring paste, then mix briefly until smooth.

Divide the batter evenly between the prepared cake pans.

Bake for 15 min, then test the center with a toothpick for doneness.

Remove the cakes from the oven and cool them completely in their pans.

Place 300 g dark cooking chocolate for ganache and 150 ml full-fat milk in a heatproof bowl.

Heat the ganache ingredients over simmering water and stir until glossy and smooth.

Trim the cooled cake edges and cut the layers to fit an 18 x 18 cm square cake ring.

Place one cake layer inside the cake ring and spread over a layer of ganache.

Add the second cake layer and cover the top with the remaining ganache.

Allow the cake to set, then cut into portions and top with chocolate flakes.