Preheat the oven to 375°F. Line a 8x6 glass baking dish with parchment paper.
In a medium bowl, whisk together the millet flour, oat flour, sweet rice flour, tapioca starch, xanthan gum, salt, baking powder, and brown sugar.
Add the vanilla and unsalted butter and use your fingers to work the butter into the flour, until it becomes a crumbly dough.
Press the dough into the parchment paper and bake until set and begins to turn golden brown, about 14-17 minutes. Let cool for 10-15 minutes.
Meanwhile, beat egg in a bowl. Whisk in the tapioca starch until completely combined.
Add the maple syrup, dark brown sugar, melted butter and vanilla to the eggs and mix thoroughly.
Sprinkle the chopped walnuts in the par-baked shortbread-lined baking dish.
Evenly pour egg mixture over walnuts.
Bake until the filling is completely set, about 20-25 minutes. If the center of the filling still jiggles a bit when the pan is moved, then return to the oven until set, about 5 minutes longer.
Let cool completely before slicing and serving.
This nutritional information has been automatically calculated, and as such, may be incomplete or inaccurate. Please reference the specific ingredients you use for the most accurate nutritional information.