Prepare the pan: Cut a sheet of parchment paper into a circle to fit the bottom of an 8 or 9-inch springform pan. Place the parchment circle on the bottom and butter the entire pan, including the parchment paper.
Melt the chocolate and butter: Fill a large saucepan halfway with water and bring to a simmer. Once the water is simmering, turn off the heat and place a large heat proof bowl over the saucepan. Add the chocolate and butter to the bowl, and stir until the chocolate is fully melted. (You can also melt the chocolate in the microwave in a liquid measuring cup, heating for a minute and then stirring, several times until the chocolate and butter are melted.)
Finish batter and pour in pan: Once the chocolate is melted, remove the bowl to the counter and slowly whisk in the maple syrup, vanilla extract, cocoa powder, and kosher salt. Whisk in the eggs and egg yolk until fully incorporated. Pour the batter into the prepared pan and smooth into an even layer with a spatula.
Prepare a water bath / bain marie (optional): Wrap aluminum foil tightly around the pan. Place it in a large pan (like a 9 x 13″ baking dish) and pour boiling water around it. The step ensures that the middle of the cake won’t sink in slightly. If you don’t feel like messing with it, you can skip this step: it looks just as good without.
Bake: Bake 30 to 35 minutes until a thin crust forms on the cake and the middle is still slightly wobbly. The internal temperature should be 150F.
Cool: Cool in the pan on a cooling rack until it is room temperature, about 1 hour. You can serve immediately or chill for 1 hour to get a more fudgy texture (that’s our favorite way to serve it). Store in fridge up to 3 days. (You can also make it ahead and freeze it; it freezes well)
Serve: Invert the cake onto a plate, then gently remove the parchment from the bottom. Revert the cake onto a serving platter and dust with cocoa powder. Dust with powdered sugar and serve with Raspberry Compote.