Preheat the oven to 350 degrees F. In a food processor, grind the almonds until fine but not powdery. Melt the stick of butter in a double boiler with the chocolate, stirring to combine, then let cool slightly.

Grease a 9-inch spring form pan with butter, and sprinkle with the breadcrumbs. Tap around to coat the bottom and sides of the pan with the breadcrumbs, and tap out any excess.

In a mixer fitted with the paddle attachment, on medium-high speed, whisk the egg yolks and ¾ cup of the sugar until thick and pale golden in color, about 2 minutes. Beat in the orange zest, the amaro or rum, and the salt. On 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, then gently fold in the rest. Spread into the prepared spring form pan, and bake until the top is firm and crackly (a toothpick will still come out with some thick batter on it), about 35 to 40 minutes.

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