Mix 1/3 cup warm water and 2 T sugar in small bowl. Sprinkle dry yeast over mixture and stir to dissolve. Let mixture stand until foamy, about 10 minutes.
Mix flour, lemon zest and salt in large bowl. Add 10 tablespoons unsalted butter and rub in with fingertips until mixture resembles coarse meal. Add yeast mixture and egg and stir until soft dough forms. Turn out dough onto lightly floured surface. Knead dough until smooth, about 5 minutes. Grease large bowl. Add dough, turning to coat. Cover dough with plastic wrap and let rise in warm draft-free area until almost doubled in volume, about 1 1/2 hours.
Lightly dust two 12-inch pizza pans or heavy large baking sheets with flour. Punch dough down and divide in half. Roll out each piece on lightly floured work surface to 12-inch diameter round. Transfer each dough round to prepared pan. Brush with melted butter. Sift 2 tablespoons powdered sugar over each. Let stand 1 hour.
Preheat oven to 425°F Bake galettes until golden brown, about 12 minutes (galettes will not rise). Cool slightly. Sift additional powdered sugar over galettes and cool completely. Cut galettes into wedges and serve.