Gâteau Opéra

Beat the almond flour, icing sugar, and whole eggs with a whisk or mixer until the batter is ribbon-stage — pale, voluminous, and falling from the whisk in a wide, sustained ribbon. Sift in the all-purpose flour and fold in. Separately, whip the egg whites to soft peaks, rain in the caster sugar, and whip to a stiff meringue. Fold the meringue into the almond batter in three additions, then fold in the cool melted butter. Spread thinly across two large, parchment-lined baking sheets to a 5 mm thickness. Bake at 200 C / 390 F for 8-10 minutes until just golden and springy. Cut into three equal rectangles for assembly.

Dissolve the espresso powder in the hot water and stir in the simple syrup until fully combined. Allow to cool to room temperature. The soak should taste intensely of coffee and sweet — stronger than you would enjoy as a drink, because the joconde absorbs it and dilutes it. The soak must be at room temperature or cooler when applied — a warm soak will damage the texture of the assembled layers.

Beat the softened butter on medium speed until very pale and fluffy — 3-4 minutes. Dissolve the espresso powder in a teaspoon of hot water to a paste, and add it to the butter. Beat in the icing sugar a third at a time, beating well between additions. The finished buttercream should be very pale (the coffee lightens as it aerates), extremely smooth, and stiff enough to hold a clean edge when spread. If it looks curdled, beat longer — the warmth of friction will smooth it.

Finely chop the dark chocolate and place in a heatproof bowl. Heat the double cream to just below simmering — small bubbles at the edge, not a rolling boil. Pour the hot cream over the chocolate in one go. Leave undisturbed for 1 minute, then begin stirring from the centre outward in small circles, gradually widening as the ganache emulsifies. It will seize at first, then suddenly become smooth and glossy. Allow to cool at room temperature, stirring occasionally, until it thickens to a spreadable but not stiff consistency.

Line a flat tray with parchment. Layer 1: place a joconde rectangle down and brush generously with coffee soak until visibly dark and moist. Spread an even 3-4 mm layer of chocolate ganache. Layer 2: place the second joconde sheet, brush with soak, and spread an even 3-4 mm layer of coffee buttercream. Layer 3: third joconde sheet, final brush of soak, and a second layer of ganache. The top must be perfectly flat — press a clean baking sheet on top gently and refrigerate for at least 1 hour to set the layers solid before glazing. Each layer must be even and the overall structure must be perfectly level.

Melt the dark chocolate gently over a bain-marie or in short microwave bursts. Stir in the neutral oil until completely smooth and fluid — the oil prevents the glaze from setting too hard and gives it a mirror finish. Allow to cool to 35-38 C / 95-100 F — just above body temperature — before pouring. Remove the assembled cake from the refrigerator and set on a rack over a tray. Pour the glaze over in one smooth motion from the centre outward, tilting the rack if needed to coat the entire surface in a single fluid layer. Refrigerate for 20 minutes until the glaze is set and mirror-bright. Trim all four edges with a hot, clean knife to reveal the cross-section of perfect layers. Dust with gold leaf or write 'Opéra' in chocolate if desired.