Preheat the oven to 350ºF and adjust the rack to the middle position.
Place the almond flour, coconut flour, gelatin, sugar, and arrowroot flour in the bowl of a food processor and pulse 4-5 times to incorporate. Add the butter and pulse for eight 1-second pulses, or until the mixture is like wet sand. Process agin until the mixture forms a firm dough.
Spoon the dough into a buttered 8×8-inch baking dish and evenly press to cover the bottom of the pan. Bake for 15 minutes, or until the crust is just turning golden brown on the edges. Let cool while you make the lemon and cranberry layers.
For the cranberry layer:
Place the cranberries and sugar in a medium sauce pan and heat over medium. Stir occasionally, and cook until the berries burst and the mixture is the texture of jam, about 15 minutes. Add the lemon zest. Pour into a blender and blend until smooth. Pour over the baked crust and spread evenly.
For the lemon layer:
Place the melted butter, honey, eggs, yolks, lemon zest, just and salt on the top of a double boiler (or place a heatproof glass bowl over a small pot of simmering water). Whisk continuously until the mixture turns thick, like a pudding, about 6 minutes. Immediately pour the lemon mixture evenly over the cranberry layer to create 2 distinct layers.
Bake for 23 minutes, or until filling is set on the edges. It will still jiggle a bit in the middle when shaken, but don’t worry — the filling will set as it cools. Cool to room temperature and then refrigerate until cold. Dust with powdered sugar, cut into bars and serve.
