Preheat oven to 325 degrees F (160 degrees C) and set a rack on the lower or middle level.
Butter a 9-inch springform pan. Line the bottom of the pan with a circle of parchment paper.
Grind the Unblanched Whole Almonds (2 cup) in a food processor, in three batches, pulsing each batch with two tablespoons of the Granulated Sugar (3/8 cup) using six tablespoons in all.
In a bowl over simmering water in the microwave, melt the Bittersweet Chocolate (8 ounce) and Unsalted Butter (1 cup) together, then set aside.
Separate Egg (6) Yolks and Whites, and set aside.
Beat the Egg Yolks with an electric mixer until lemon-colored, about 5 minutes. Gradually beat in Granulated Sugar (7/8 cup) (3/4 cup + 2 Tbsp).
Add the chocolate mixture, stirring to mix well. Add the ground almonds and stir to incorporate. Add the Sea Salt Flakes (1/2 teaspoon) and Grand Marnier (2 tablespoon).
In a clean bowl, beat the Egg Whites with the remaining quarter cup of sugar until they form firm peaks. In two additions, fold the egg whites into the chocolate mixture.
Pour the batter into the prepared cake pan and smooth the top. Put the pan on a cookie sheet and bake for 90 minutes.
Let cool in the pan on a wire rack for 15 minutes before removing the sides of the springform pan, then cool completely.
When the cake has cooled, turn it upside down onto a serving plate. Peel off and discard the parchment paper. Just before serving, sift Powdered Confectioners Sugar (to taste) over the top.