Preparation

• Position a rack in the center of the oven and heat the oven to 350°F. Butter the bottom and sides of a 9-inch springform cake pan.
• Combine the chocolate and butter in a large bowl set in a skillet of barely simmering water. Stir occasionally until melted and smooth, about 5 minutes. Remove the bowl from the skillet and set aside.
• Pulse the almonds in a food processor until finely chopped, about 10 one-second pulses; set aside 1/4 cup. Add 1/4 cup of the granulated sugar to the almonds remaining in the processor and pulse until the mixture resembles damp sand, another 10 pulses. (Don’t overprocess or the mixture will become oily.) Stir into the chocolate along with the rum and vanilla.
• In a stand mixer fitted with the paddle attachment (or in a large bowl using a hand-held electric mixer), combine the remaining 1/2 cup sugar with the eggs and salt. Beat on medium-high speed until tripled in volume and the lifted beater leaves a ribbon on the surface of the mixture, about 5 minutes.
• Using a spatula, fold the chocolate-almond mixture into the eggs until no streaks remain. Transfer the batter to the prepared pan and sprinkle with the reserved almonds.
• Bake until the top is set and the cake doesn’t jiggle when you gently shake the pan, 25 to 30 minutes. Cool on a rack for 30 minutes. Run a knife around the edge of the pan, and remove the side. Use a large, lightly oiled spatula to carefully transfer the cake to a serving plate. Cool completely before slicing and serving.