Preheat the oven to 350F and line an 8x8 inch pan with parchment paper. Mix the almond flour, 3 tbsp powdered allulose, salt, melted butter and 1/2 tsp vanilla until it clumps like wet sand. Press firmly and evenly into the pan and bake for 12 minutes until lightly golden.----------------
While the crust bakes, whisk together the egg yolks, whole egg and 1/2 cup powdered allulose until smooth. Whisk in the lemon juice, lemon zest and heavy cream, then whisk in the coconut flour until no lumps remain.----------------
Pour the lemon filling over the hot crust and return to the oven. Bake for 18 to 20 minutes, until the filling is set around the edges with just a slight jiggle in the center.----------------
Cool at room temperature for 30 minutes, then refrigerate for at least 2 hours until fully chilled and firm.----------------
In a very clean bowl, beat the egg whites with the cream of tartar on medium speed until foamy. Gradually add 1/2 cup powdered allulose, one spoonful at a time, then increase to high speed and beat until glossy, stiff peaks form, about 4 to 5 minutes. Beat in 1/2 tsp vanilla.----------------
Pile the meringue over the chilled lemon layer and use the back of a spoon to create swoops and peaks all over the top.----------------
Toast the meringue with a kitchen torch until golden, or place under the broiler for 1 to 2 minutes, watching constantly. Let cool for 10 minutes, then lift out and slice into 12 bars. Store in the fridge and enjoy within 2 days.