In a medium saucepan over medium heat, combine 2 cups blackberries and lemon juice.
Bring to a boil and then reduce the heat and simmer for 10 minutes, until the berries are soft.
Take sauce pan off the heat and whisk in swerve and xanthan gum and smash the softened blackberries. Stir in remaining 1 cup of blackberries until combined. Set aside while you prepare the crust.
Preheat the oven to 350 degrees. Line an 8 inch square baking dish with parchment paper, leaving paper overhanging 2 sides of the pan for easy removal.
Whisk together the almond flour, confectioner’s swerve, and salt; then add the butter and vanilla and use your hands to combine the ingredients until the mixture is combined and in pea-sized crumbles.
Take half of the dough and press it evenly into the bottom of the pan. Par-bake the crust for 12 minutes, until just beginning to brown.
Pour the filling over the par-baked crust and then crumble the remaining dough evenly over the top of the filling. Bake for 22-25 minutes until the topping is golden brown. Remove from the oven and let it cool completely.
Use the overhanging parchment paper to lift the bars out of the pan, then slice into 9 equal sized squares.
Enjoy chilled or at room temperature. Store leftovers in the fridge for up to a week.