Preheat the oven to 150C / 300F. Using an 8-inch cake pan as a guide, trace three 20 cm circles onto parchment paper, then flip the paper ink-side down onto two 13 x 18-inch sheet pans.

In a large bowl, whip the egg whites and salt with a mixer on medium speed until soft peaks form, about 2 minutes.

Add the sugar one tablespoon at a time, whipping after each addition, until the meringue is stiff and glossy, about 5 minutes more.

Fold in the ground walnuts and vanilla with a spatula in three additions, working gently so you don't deflate the meringue.

Divide the batter evenly among the three traced circles and spread to the edges with an offset spatula, keeping the layers about 1 cm thick.

Bake for 22 to 25 minutes, rotating the pans halfway, until the layers are light golden and dry to the touch. Cool completely on the pans, at least 20 minutes, the layers crisp as they cool and will tear if peeled while warm.

Whisk the egg yolks and sugar together in a 2-quart / 1.9L saucepan until pale, then whisk in the milk.

Cook over low heat, stirring constantly, for 6 to 8 minutes until the custard coats the back of a spoon and reaches 82C / 180F. Pull it off the heat right away, if it scrambles at the edges you've gone too far.

Pour the custard into a bowl and press plastic wrap directly onto the surface. Cool to room temperature, about 30 minutes.

Beat the softened butter in a large bowl until pale and fluffy, 2 to 3 minutes. Add the cooled custard a spoonful at a time, beating well after each addition.

Beat in the melted dark chocolate, rum, and vanilla until the buttercream is smooth and holds its shape.

Set one walnut layer on a cake plate. Spread a third of the buttercream evenly over the top. Repeat with the second and third layers, using the last of the buttercream to coat the top and reserving enough to thinly coat the sides.

Press the reserved 50 g ground walnuts into the sides of the torta with your palm.

Melt the chopped chocolate and cream together in short bursts in the microwave, stirring until smooth, then stir in the butter for shine. Pour over the top of the torta and let it drip slightly over the edge.

Chill the torta for at least 4 hours, or overnight, until the buttercream is firm enough to slice cleanly. Warm your knife under hot water and wipe between cuts.