Heat the oven to 350°F. Line an 8x8-inch pan with parchment, leaving an overhang on two sides.

Chop the baking chocolate and melt it with the butter in a small saucepan over low heat, stirring until smooth. Let it cool for 5 minutes.

In a large bowl, beat the eggs and granulated erythritol with a mixer or strong whisk for 2 minutes, until pale and thickened. Beat in the vanilla.

Stream in the melted chocolate while mixing until glossy.

In another bowl, whisk the almond flour, cocoa powder, baking powder, and salt. Fold into the chocolate mixture until no dry streaks remain. Chop the walnuts and fold in.

Spread the batter evenly in the pan and bake 20 to 22 minutes, until the top is set and crackly and a toothpick in the center comes out with moist crumbs.

Cool completely in the pan, at least 1 hour, then lift out and cut into 16 squares.