Heat the oven to 170 C / 340 F. Butter and lightly flour an 18 cm / 7-inch domed cake pan or a heatproof mixing bowl of the same size.

Toast the almonds in a dry 26 cm / 10-inch skillet over medium heat for 4-5 minutes, stirring often, until fragrant and lightly colored. Let cool.

Grind the cooled almonds with 2 tbsp of the sugar in a food processor until fine and sandy, not pasty. Tip into a bowl and stir in the semolina, flour, and salt.

In a separate bowl, whisk the egg yolks with the remaining sugar for 2-3 minutes until pale and thick. Whisk in the melted butter, lemon zest, and almond extract.

Fold the almond mixture into the yolk mixture in two additions until just combined.

In a clean bowl, whip the egg whites to soft peaks. Fold them into the batter in three additions, working gently to keep the air in.

Pour the batter into the prepared pan and smooth the top. Bake for 35 to 40 minutes, until a skewer inserted in the center comes out clean and the internal temperature reads 200 F / 93 C.

Cool the cake in the pan for 10 minutes, then turn it out onto a wire rack. Let it cool completely, at least 2 hours, before coating, since a warm cake will make the chocolate slide and set dull instead of glossy.

Trim the flat base if needed so the cake sits steady once flipped dome-side up on the rack, with a tray underneath to catch drips.

Melt the chocolate and optional butter in a double boiler or in the microwave in 20-second bursts, stirring until smooth and pourable, about 45 C / 113 F.

Pour the chocolate over the dome, using an offset spatula to coat the sides evenly. Let the excess drip through the rack.

Let the coating set at room temperature for 30 to 40 minutes, or in the fridge for 15 minutes, before slicing with a warm, dry knife.