Preparation Put skillet on middle rack of oven and preheat oven to 450°F. Stir together sugar and zest in a small bowl. Beat eggs with an electric mixer at high speed until pale and frothy, then beat in milk, flour, vanilla, cinnamon, nutmeg, and salt and continue to beat until smooth, about 1 minute more (batter will be thin). Add butter to hot skillet and melt, swirling to coat. Add batter and immediately return skillet to oven. Bake until puffed and golden-brown, 18 to 25 minutes. Serve immediately, topped with lemon sugar.
Preparation Put skillet on middle rack of oven and preheat oven to 450°F. Stir together sugar and zest in a small bowl. Beat eggs with an electric mixer at high speed until pale and frothy, then beat in milk, flour, vanilla, cinnamon, nutmeg, and salt and continue to beat until smooth, about 1 minute more (batter will be thin). Add butter to hot skillet and melt, swirling to coat. Add batter and immediately return skillet to oven. Bake until puffed and golden-brown, 18 to 25 minutes. Serve immediately, topped with lemon sugar.