Preheat the oven to 350 degrees.

In a food processor, grind the almonds until fine but not clumpy. Grease a 9-inch springform pan with butter, and sprinkle with the breadcrumbs. Coat the bottom and sides of the pan with the breadcrumbs, and tap out any excess. Combine the chocolate and 1 stick butter in a double boiler set over simmering water, and melt over low heat. Stir to combine, and let cool slightly.

In an electric mixer fitted with the paddle attachment, whisk the egg yolks and ¾ cup of the granulated sugar at medium-high speed until thick and pale gold in color, about 2 minutes. Beat in the orange zest, the rum or amaro, the vanilla, and the salt. At low speed, beat in the cooled chocolate mixture just until smooth. Fold in the ground almonds.

In the mixer, in a clean bowl fitted with the whisk attachment, whisk the egg whites and remaining ¼ cup sugar to stiff peaks. Stir a third of the whites into the chocolate mixture to lighten it, then gently fold in the rest. Spread into the prepared pan, and bake until the top is firm and crackly (a toothpick will come out with some thick batter still on it), about 35 to 40 minutes.

Let the cake cool on a rack for 5 minutes, run a knife around the edge to loosen it, and unmold. Serve sprinkled with confectioners’ sugar.