Preheat your oven to 325 degrees (F). Using a 7-inch (or 9-inch) springform pan, place a sheet of parchment paper on the bottom, and close the “ring” of the pan around the paper. Use a scissor to trim all the extra paper around the outside of the pan. Then spray the sides and bottom with non-stick cooking spray.
Chop your chocolate bars into small pieces. Place in a microwave-safe bowl. Add the butter, and set aside.
Separate the eggs. Place the yolks in a mixing bowl and add the powdered sweetener. Mix well until the sweetener and yolks have completely combined.
Place the egg whites in a stand mixer with the whisk attachment (or large bowl with an electric hand mixer). Beat the whites on high speed until they form stiff peaks. Set aside.
Place the bowl with the chocolate and butter in the microwave. Microwave on high for 1 minute. Stir the contents, and then microwave for 30 seconds more. Stir well, using a rubber spatula, and ensure all the chocolate is melted and completely smooth.
Temper the chocolate with the yolks, by adding a bit of the warm chocolate to the egg yolk mixture, and beat very quickly with a spatula. Then add the rest of the chocolate, and again, mix aggressively. Move quickly so you don’t scramble the yolks with the warm chocolate.
Add 1/3 of the egg whites to the chocolate mixture and mix well. Add another third of the egg whites, but this time, very gently fold the chocolate with the egg whites. Once combined, add the last third of the whites, and gently fold once last time until a uniform color and light, airy batter is formed.
Carefully pour the batter into the pan. Tap the pan to remove any bubbles, and place in the middle rack of the oven.
Look for the center to be puffy, glossy, but still “giggly” if you shake the pan. Remove from the oven, and let it cool completely before removing the cake from the pan.
Once cooled, dust the top with powdered sugar. It’s delicious on its own, or serve with vanilla ice cream or fresh berries.
Note: It’s best to serve this dish the same day you bake it, as the gooey center will continue to firm up over time.