Preheat oven to 350ºF (180ºC).
Lightly butter or spray a 10-inch round cake springform pan.
Melt 8 ounces semisweet chocolate in a glass bowl or heat-proof bowl over a pan of boiling water, being careful that the bottom of the bowl does not touch the water. Or microwave it (in a microwave-safe bowl) in 30-second intervals, mixing well between each one and being careful not to scorch the chocolate. Let cool to room temperature.
Grind 2 cups walnuts with ¼ cup of the sugar. Transfer to another bowl.
Using the same processor, puree 1 ½ cups beans with 2 tablespoons dark rum and 1 tablespoon vanilla extract.
Separate 6 eggs, put yolks in a small bowl and whites in a clean, dry large bowl.
Add the egg yolks, one at a time, and process until smooth. Add the melted chocolate and process until smooth. Don't overprocess.
With an electric mixer, beat egg whites and pinch salt until soft peaks start to form. While beating at high speed, gradually add the remaining ¾ cup sugar and beat until stiff peaks form and the meringue is glossy.
Add ¼ of the chocolate batter into the meringue, and mix lightly with a rubber or silicon spatula. Add the rest of the chocolate in two more additions, mixing lightly between each one. It will not be even colored.
Add the ground walnuts and mix with a spatula until it's all one even-colored batter, but careful not to overmix.
Put into the prepared pan, place an aluminum foil in the bottom of the oven if using a springform pan in case the cake drips.
Bake for about 50 minutes to 1 hour, until the top is firm and a cake tester or toothpick inserted in the center comes out clean. Transfer to a cooling rack until the cake is at room temperature.
Remove the springform ring and transfer the cake to a serving plate, dust with 2 teaspoons powdered sugar and serve.
Store leftovers in the fridge, wrapped in plastic or in an airtight container and return to room temperature before eating.