Preheat oven on 325 degrees for convention ovens with fan (350 degrees without fan)
Spray a 9-x-9-inch cake pan with cooking spray and line with parchment paper
Combine flour and salt and set aside.
Combine chocolates and butter in a mixing bowl and melt over a low-heat double boiler or very carefully in the microwave. Stir to combine and set aside.
Combine the whole eggs and yolks in a mixing bowl and whisk until lightly frothy, add stevia and continue to whisk to combine.
Add vanilla extract and espresso and stir to combine.
Add chocolate and butter mixture to the eggs whole stirring. Stir until combined.
Add flour and salt mixture and mix just until fully combined.
Spread into prepared pan.
Bake in preheated oven for 10-12 minutes.
Let cool for 15 minutes before digging in.
Keep refrigerated for best storage.