Preheat the oven to 160c and butter a 23cm springfrom pan, Melt the butter and chocolate together in the microwave. In the meantime, whisk the flour, cocoa powder and salt in a bowl.
In a large bowl, beat eggs, sweetener, and vanilla until smooth. Beat in almond flour mixture, then butter/chocolate mixture until smooth. Stir in nuts.
Spread evenly over the bottom of the prepared pan. Bake 12 to 18 minutes until set around the edges but still soft in the center. Let cool for 15 to 20 minutes.