Set your oven to 170°C. Line a 16 cm baking pan with parchment paper.

Whisk 35g hot milk and 15g cocoa powder until smooth.

Beat 3 eggs and 85g sugar until combined.

Place the egg mixture over simmering water and heat to 40°C while whisking.

Place the bowl of cocoa-milk mixture and 15g butter into the warm saucepan to melt.

Use an electric mixer to beat the egg mixture at high speed for 6 minutes, then low speed for 1 minute.

Sift 75g flour in two batches, folding gently into the egg mixture.

Fold the flour mixture into the melted cocoa butter mixture.

Pour batter into the pan, tap out air bubbles, and bake at 170°C for 40 minutes.

Mix 75ml hot espresso and 10g sugar, adding 15 ml rum if desired.

Melt 110g milk chocolate with 110g cream, then fold in 350g cold cream and chill.

Once cooled, slice the cake into 4 layers, each 1 cm thick.

Whip the chilled chocolate buttercream over an ice bath until soft peaks form.

Brush each layer with coffee syrup, top with whipped cream, and repeat with the other layers.

Whisk the reserved cream and frost the cake, smoothing with a spatula and piping a decorative border. Chill the frosted cake for at least 3 hours.

Melt 27g chocolate chips and 27g cream, then pour over the cake and chill for 15 minutes.

Let the cake set, slice, and enjoy the rich, decadent result!