In a large bowl, place the flour, xanthan gum, baking powder and salt, and whisk to combine well. Add the chopped and chilled butter, and toss to coat it in the dry ingredients. Flatten each chunk of butter between your thumb and forefinger. Add the sour cream, and mix to moisten the dry ingredients with the sour cream. The dough should be shaggy and somewhat crumbly. Knead the dough together with clean hands until it begins to come together. Add ice water by the teaspoon only if necessary for the dough to hold together. Turn the dough out onto a sheet of plastic wrap, and press into a disk as you close the plastic wrap around the dough. It will still seem rough. Place the dough in the refrigerator to chill for an hour.
Preheat your oven to 375°F. Grease a 9-inch pie plate with butter and set aside.
Once the dough has chilled, turn it out onto a lightly floured piece of unbleached parchment paper. Sprinkle the dough lightly with more flour, and roll it out into a rectangle that is about 1 inch thick, moving the dough frequently and sprinkling it lightly with flour if it begins to stick. Fold the dough over on itself, sprinkle the dough again lightly with flour, and roll out the dough once again into a rectangle about 1 inch thick. Complete the folding and rolling process once more, finally rolling out the dough into an approximately 12-inch round, a little less than ½ an inch thick.
Roll the pie crust loosely onto the rolling pin and then unroll it over the prepared pie plate. Trim the roughest edges of the crust with kitchen shears. Lift up the edges of the pie crust gently to create slack in the crust, and place the crust into the bottom and up the sides of the pie plate. Tuck the excess pie crust under itself, and crimp the edge gently all the way around the crust by pinching the dough at regular intervals with one hand, and creating a crimped impression with the forefinger of the other hand. If you are unable to do this, just leave the edges as they are after folding over the excess - either way it's beautiful! Cover the pie crust with plastic wrap and place it in the refrigerator to chill until firm, at least 30 minutes.
Preheat oven to 350° F while pie crust is chilling. Once pie crust is chilled, remove the plastic wrap and prick the bottom of the crust several tines with a fork. Place a piece of parchment over top of of the crust and place dried beans or pie weights over the bottom the crust. Parbake crust for 8-10 minutes, just until edges start to turn golden.
While pie crust is par baking, combine butter, maple syrup, sugar, eggs, heavy cream, vanilla, and salt in a large bowl. Whisk until well combined, then add in sifted almond flour and stir until incorporated.
Pour filling into partially baked crust, and bake for an 40-45 minutes, until the center of the pie is set and is a light golden brown color. I recommend loosely covering the edges of the pie with foil for at least half of the cooking time to avoid overcooking the crust. This crust can become tough if overcooked.
Remove eggs from the refrigerator and separate whites while cold. Place egg whites in a small bowl, cover and allow to come to room temperature. Complete this step before making pie filling to save time.
Pour eggs whites into a spotlessly-clean bowl. Whip the egg whites until they are glossy and smooth, then add the cream of tartar to help the egg whites hold their form and mix for another minute to incorporate.
Slowly add the sugar to the egg whites about a tablespoon at a time, beating well after each addition until the sugar is incorporated. Continue beating until stiff peaks form.
Spread meringue on top of warm pie filling, all the way to the crust, and use the back of a spoon to create peaks by lightly pressing spoon into meringue pulling away quickly. Place pie back in the oven on the middle rack for about 10 minutes until meringue is lightly toasted and golden around the edges.