Preheat the oven to 350°F and line an 8x8-inch baking pan with parchment paper, leaving overhang on two sides.
Mix the almond flour, erythritol, salt, melted butter and vanilla until it looks like wet sand. Press firmly into the pan and bake for 10 to 12 minutes, until golden at the edges. Cool completely.
Sprinkle the gelatin over the cold water to bloom. Whisk the xanthan gum into the powdered erythritol, then whisk this with the egg yolks until pale.
Heat the heavy cream and almond milk until steaming, then slowly whisk it into the yolk mixture. Return everything to the saucepan.
Cook over medium-low heat, stirring constantly, for 5 to 6 minutes until thick enough to coat the spoon. Off the heat, stir in the gelatin, butter, banana extract and vanilla until smooth.
Let the custard cool for 10 minutes, then pour it over the crust. Chill for 3 hours, until fully set.
Whip the topping cream with the erythritol and vanilla to stiff peaks and spread it over the custard. Lift out and slice into 16 bars.