Preheat the oven to 325 degrees F (163 degrees C). Line a 8x8 in (20x20 cm) glass baking dish with foil or parchment paper and grease lightly. Clip the edges of the liner to the pan with plastic clips. (This is temporary. They don't need to be oven safe.) Set aside.

Melt the butter and chocolate in a double boiler on the stove. (Boil water in a pot, place the butter and chocolate into a heat proof bowl, and place on top of the pot. Heat, stirring occasionally, until melted.) Set aside to cool for a couple of minutes. (You can also do this in the microwave if you prefer. Be careful not to burn it.)

Place the avocado, eggs, and vanilla in a high-power blender or food processor. Puree until smooth. Add the melted butter/chocolate mixture and puree again.

In a small bowl, stir together the almond flour, cocoa powder, and sweetener. Add the dry ingredients to the blender or food processor and stir in with a spatula. Pulse a few times until just combined, scraping down the sides as needed. (Don't overmix.)

Spread the batter into the prepared baking pan. It will be thick, so the clips on the parchment or foil will help. Smooth the top with a spatula. Remove the clips.

Bake about 20-30 minutes, until brownies are barely set. The top should no longer be wet, but still be very soft. An inserted toothpick will come out with just a small amount of batter on it. Cool completely to firm up before cutting.