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To make the crust: Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess. Whisk together the flour, salt, and baking powder. Cut the cold butter into pats, and work it into the flour mixture until it's unevenly crumbly, with larger bits of butter remaining intact.
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Stir in the sour cream and water. The dough will be craggy but cohesive. Turn the dough out onto a well-floured surface and bring it together, if necessary, with a few quick kneads.
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Pat and roll the dough into a rough 8" x 10" rectangle. Make sure the underside is sufficiently dusted with flour that you can move it around easily.
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Starting with one of the shorter (8") ends, fold the dough in thirds like a business letter. Flip it over (so the open flap is on the bottom), and turn it 90°. Roll the dough into an 8" x 10" rectangle again. Fold it in thirds again.
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Wrap the dough in plastic, and place it in the refrigerator to chill for at least 30 minutes (or overnight).
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When you're ready to proceed, start preheating the oven to 400°F. Lightly grease a baking sheet, or line it with parchment.
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Divide the pastry in half. Roll one half into a 10" square. Using a 10"-round template (e.g., a dinner plate), cut a 10" circle. Set the circle onto the prepared baking sheet.
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Roll the other piece of pastry into an 11" square, and cut an 11" circle. Set it aside.
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To make the filling: Beat the almond paste, butter, sugar, and salt until thoroughly combined. Beat in the egg yolks and vanilla, then stir in the flours.
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To make the glaze: Mix the egg yolk and water together.
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To assemble the galette: Spread the filling over the 10" circle on the baking sheet, leaving a 1" rim around the edge of the pastry. Brush some glaze over the uncovered edge of the pastry.
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Center the 11" circle of dough over the filled bottom crust, and smooth it over the filling. Using a fork, press and crimp the edge of the galette to seal.
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Decorate the galette by using the back of a knife to trace a pattern on the surface; you'll just barely cut into the surface without cutting all the way through. Poke a vent hole in the center, and four additional small slits at other random spots, hiding the slits in the pattern you've drawn. Brush an even coat of the glaze over the entire surface of the galette.
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Bake the galette for 30 to 35 minutes, or until it's golden. Don't be afraid to let it become deeply browned; this slight caramelization gives the butter in the crust wonderful flavor. Remove the galette from the oven, and cool it slightly right on the baking sheet.
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Serve galette warm or at room temperature. Store any leftovers at room temperature for several days; freeze for longer storage.
