Procedure:

1) Place half the buckwheat groats in the spice grinder. Run grinder for 10 seconds, stopping at 5 seconds to shake and swirl. Pour ground buckwheat into the large bowl and do the same with the second half. Pick through and remove any whole groats.

2) Put the egg yolk in a calibrated measuring cup. Add  the vinegar, then enough cold water to reach a scant 1/3 cup. Whisk to blend and then place into freezer while preparing flour and butter.

3) Add the salt, sugar and xanthan gum to the flour. Whisk until well blended. Cutting across the butter stick, slice butter into slabs 1/8 inch thick, and toss into the flour blend. Stir lightly to blend, then turn out onto a plastic wrap covered work area. Use the heel of your hand to smear the butter into the flour, making 3 or 4 passes.  Spread flour/butter blend out into a rectangle about 6 inches X 10 inches, 2 inches thick.

4) Pour the egg/water/vinegar mixture onto the flour/butter rectangle, moving slowly and lifting the plastic from underneath if liquid tries to escape. Layer plastic wrap over the rectangle and tap/pound the plastic from above to hydrate the flour. Uncover the dough, place a hand beneath the plastic and do a "Letter fold", bringing 1/3 of the dough up to cover the middle third of dough and then folding the remaining third over the previous two. Recover the dough, tap and pound from above to spread the dough into a rectangle, and do a second letter fold. Do this two more times, or until the dough is hydrated throughout. Wrap in the plastic wrap and refrigerate for a minimum of 2 hours, or overnight. 

5) Remove dough from refrigerator and place onto a well-dusted work surface. Heat oven to 425. Roll dough into a slab that is 1/8 inch thick. If dough is too resistant, allow to warm slightly. You will need to slide a spatula under the dough from time to time to prevent sticking. Cracks can be repaired while rolling. Cut dough into a circle or circles to fit pan(s). Use a wide sheet of thin cardboard to transfer dough to pans. Carefully press dough into pans and pinch decorative marks into the edge. Bake for 12 minutes and remove to a cooling rack. Reduce oven heat to 350. Use thin strips of aluminum foil around edges to protect from over baking. 

6) Make the filling: Place the almonds into the spice grinder and run for 10 seconds. Shake grinder then run another 10 seconds. Pour the resulting flour into a medium bowl, scraping out any that adheres to the spice grinder. Melt the butter - 30 seconds in a microwave on high. Separate the eggs, allowing a small amount of one egg white to drool into the yolk bowl. 

7) Stirring with a spoon, add to the flour in this order:  the sugar, the melted butter, the egg. Stir until smooth. Add the brandy and the almond extract and stir to blend. Pour into pie shell. Bake for 10 minutes.

8) Meanwhile, place the reserved almond into the container used to microwave the butter and stir. Add the half and half and stir. Add the sugar and the reserved almond extract and stir. After the ten minutes of baking, scatter the almond flakes atop the custard and return pie to oven. Bake another 15 minutes, remove to a cooling rack, and eat as soon as it won't burn your mouth.