Preheat the oven to 375 degrees F.
Use a pastry brush to evenly coat the inside of a 1 1/2-quart souffle mold with softened butter. Fill the mold with granulated sugar, then pour out the excess. If you have properly buttered the mold, the sugar will stick to the side and bottom of it. The butter and sugar keeps the souffle from sticking to the side of the mold and allows it to rise evenly. The sugar also gives the souffle a crunchy crust.
Place a double boiler 1/2 filled with water over high heat and bring to a boil. Place the chopped chocolate in the top of the double boiler and melt until quite hot. Stir occasionally.
Place the eggs, sugar and lemon in another double boiler placed over boiling water. Whisk the egg mixture until lukewarm. Remove the egg mixture from the heat and pour into the bowl of a stand mixer. Whip to stiff peaks. While the egg mixture is whipping, check the chocolate. It should be hot to the touch. Stop the mixer. Use a rubber spatula to fold the hot chocolate into the whipped egg whites.
Use a rubber spatula to gently place the souffle mixture in the buttered and sugared mold. Fill to about 1 inch above the rim of the mold. Place the souffle in the center of the oven and allow enough room for it to rise. If the souffle is too close to the top of the oven or under a rack, it will stick when it rises. If the souffle is too close to the bottom of the oven, the bottom of the souffle will burn before the inside is properly baked. Bake until the souffle has risen 1 1/2 times in height and starts to brown on top, about 20 minutes. Remove from the oven and dust the top with the powdered sugar. Serve immediately with a side of whipped cream or Creme Anglaise.
Note: This souffle can also be baked in buttered and sugared individual size molds. Use a pastry bag with a large opening (no tip) to pipe the souffle mixture into the molds about 1 inch above the rim. Bake until risen in height and lightly browned on top, 6 to 8 minutes.