Position a rack in the lower third of your oven and preheat to 350F/180C. Line with parchment paper the bottom and sides of a 8x8-inch baking pan. Set aside.
Add butter, sweetener (stevia does not work!), cocoa powder and salt to a medium heatproof bowl.
Melt this mixture over a water bath whisking constantly (or use the microwave, in small increments). You'll want to heat it up until most of the sweetener has melted and the mixture is well incorporated (note that erythritol, unlike xylitol, won't dissolve much at this point). Remove from heat and allow the mixture to cool slightly.
Add one egg at a time, whisking well after each one until completely incorporated. The texture should appear smooth, with all the sweetener dissolving into the mixture. And if using erythritol, and your batter ends up too thick, you may want to add an extra egg. Just be sure not to over-whisk, or your brownies could end up more cakey rather than fudgy.
Add the almond flour, whisking vigorously until fully blended (about a minute).
Bake for 10-20 minutes (we do about 23 with xylitol and 17 with erythritol), or until the center is just set and a toothpick inserted in the center comes out moist. This really does vary (a lot!!) from oven to oven (think convection etc), so give them a check from minute 15 the first time around, and remember that your brownies will continue to cook while they cool.
Sprinkle with flakey sea salt (optional) and allow to cool completely on a rack. Lift brownies using the edges of the parchment paper and cut into desired size (we do 16!). To get extra clean edges, place in the freezer for 10 minutes prior to cutting.
