10 t (1-1/4 sticks) unsalted butter, melted, cooled slightly
Preheat oven to 375 degrees F. Generously butter and flour pan for large madeleines* (about 3x1-1/4 inches). Using electric mixer, beat eggs and 2/3 cup sugar in large bowl just to blend. Beat in vanilla, lemon peel and salt. Add flour; beat just until blended. Gradually add cooled melted butter in steady stream, beating just until blended.
Spoon 1 tablespoon batter into each indentation in pan. Bake until puffed and brown, about 16 minutes. Cool 5 minutes. Gently remove from pan. Repeat process, buttering and flouring pan before each batch. (Can be made 1 day ahead.)
Dust cookies with powdered sugar.
*A metal mold with scallop-shaped indentations, sold at cookware stores.