Line an 8x8-inch pan with parchment paper. Toast the almond flour and shredded coconut in a dry skillet over medium heat for 3 to 4 minutes, stirring constantly, until lightly golden. Transfer to a bowl and add the walnuts.
Melt 1/2 cup butter in a saucepan over low heat and whisk in the cocoa powder, erythritol and salt until smooth. Remove from the heat for 1 minute, then quickly whisk in the beaten egg.Return to low heat and stir for 1 to 2 minutes, until it thickens like pudding. Stir in the vanilla.
Pour the chocolate mixture over the toasted crumbs and stir until evenly moistened. Press firmly into the pan and chill for 20 minutes.
For the custard layer, beat the softened butter for 2 minutes until pale. Add the powdered erythritol, heavy cream, vanilla and salt and beat until fluffy and smooth.
Spread the custard layer over the chilled base and chill for 30 minutes, until firm to the touch.
Melt the chocolate chips with 2 tbsp butter in 30-second bursts, stirring until glossy. Let cool for 5 minutes so it does not melt the custard layer.
Pour the chocolate over the top and tilt the pan to cover evenly. Chill for 1 hour until set.
Let the bars sit at room temperature for 10 minutes so the chocolate does not crack, then slice into 16 bars.