1. Preheat oven to 350°. Butter and flour a 9-inch cake pan. In a food processor, pulse almonds until finely ground.
2. In a large bowl, with a mixer on medium-high speed, beat butter and 1/2 cup sugar until light and fluffy. Add egg yolks, one at a time, beating well after each addition. Add chocolate, vanilla, and almonds. Beat thoroughly. Gently mix in buckwheat flour.
3. In a medium bowl, whisk egg whites until foamy. Add salt and whisk until soft, glossy peaks form. Whisk in remaining 1/4 cup sugar. Gently fold one quarter of the beaten egg whites into the batter, then fold in remaining whites. Pour batter into pan.
4. Bake until a knife inserted in center comes out clean, about 30 minutes. Let cool in pan on a rack for 10 minutes, then invert out onto a plate and cool completely.
Note: Nutritional analysis is per serving.