This moist cake tastes incredibly rich, but its relatively low in fat and calories. You can easily make it a day ahead.
Preheat oven to 350F. Spray 10-inch springform pan, and line with parchment. Spray parchment, and dust with cocoa powder.
Grind almonds with 1/4 cup sugar in
food processor until it forms a smooth meal. Measure out 1 1/2 cups.
Combine cocoa powder and hot water; mix until smooth. Stir in vanilla.
Beat egg yolks with remaining sugar at high speed 5 to 8 minutes, or until thick and pale yellow. Add chocolate mixture. Mix in almond meal at low speed, in 2 or 3 additions, beating 20 seconds after each addition.
In a separate bowl, beat egg whites using an electric beater, starting on low and slowly increasing speed, until they begin to foam. Add cream of tartar and salt, and continue to beat until whites form stiff but not dry peaks. Stir one-quarter of mixture into batter, and gently fold in remainder with rubber spatula. Pour into prepared pan.
Bake 45 to 55 minutes, or until firm in center and tester comes out clean. Cool in pan on rack 15 minutes. Remove sides of pan, and cool completely on rack. To serve immediately, dust with confectioners’ sugar. If making ahead, cover tightly with plastic wrap and refrigerate until ready to use.