Preheat the oven to 325 degrees F. Line an 8-inch square glass or ceramic baking dish with parchment, leaving an overhang on 2 sides.
Bring a few inches of water to a boil in a medium saucepot. Combine the butter, chocolate chips and heavy cream in a medium heatproof bowl. Set the bowl on top of the saucepot and heat, whisking, until the mixture is melted and combined. Remove from the heat.
Whisk together the sweetener, cocoa powder, almond flour and salt in a separate medium bowl.
Add the eggs and yolk one at a time to the melted chocolate mixture, whisking after each addition until just incorporated. Whisk in the vanilla. Whisk in the dry ingredients to combine. Pour the batter into the prepared baking dish and use an offset spatula to spread it to the edges in an even layer. Sprinkle with the nuts or coconut chips if using.
Bake until the edges of the brownie are matte and completely set but the center is still slightly shiny, 18 to 20 minutes. Cool in the baking dish for about 10 minutes, then use the parchment to transfer the brownie to a wire rack to cool completely. Cut into 12 pieces.
Erythritol, a sugar substitute, can be found in most health food stores or online.
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