Step 1

Position a rack in the lower third of your oven and preheat to 350°F/180°C. Grease and line with parchment paper an 8-inch cake pan. Set aside.

Step 2

Melt chocolate and butter over a water bath, stirring frequently until smooth. Stir in cocoa powder, espresso powder (optional) and salt. If you're using sugar-free chocolate with erythritol (such as Lily's), the mixture will turn very thick- worry not! Remove from heat, stir in vanilla extract (optional) and allow the mixture to cool slightly while you prepare the rest.

Step 3

Add egg whites to a medium bowl and whisk until soft peaks forms. Be sure not to over-do it, as stiff whites will be tougher to fold in. Set aside.

Step 4

Add egg yolks and sweetener to a large bowl and beat with an electric mixer (or with a whisk and some arm muscle!) until light and fluffy (about 3-5 minutes). Add in the prepared chocolate mixture in thirds, mixing fully before incorporating the next part. If using sugar-free chocolate, add in a couple tablespoons water to loosen up the batter.

Step 5

Fold the egg whites gently in four parts, be very gentle as the batter will lose up as you go.

Step 6

Pour into prepared pan and bake for 23-26 minutes for fudgiest results and 27-30 for cakey edges. Or until the center is just set and a toothpick inserted in comes out moist (not drenched). This really does vary (a lot!!) from oven to oven (think convection etc), so give it a check from minute 15 the first time around, and remember that you're cake will continue to cook as it cools.

Step 7

Allow to cool completely, and to get extra clean edges place in the freezer for 10 minutes prior to cutting. Dust with cocoa right before serving and store in an airtight container at room temp for 3-5 days (or in the freezer for a month or two).