2 2/3 cups cake flour
3/4 teaspoon double-acting baking powder
Zest of 3 lemons - removed with a zester and very finely chopped
2 cups sugar
6 large eggs, at room temperature
3/4 cup crème fraîche, homemade or store-bought, at room temperature, or heavy cream
31/2 tablespoons rum
Pinch of salt
1 stick plus 1 tablespoon (4 1/2 ounces) unsalted butter, melted and cooled
About 1 cup lemon marmalade, for glaze, optional
Center a rack in the oven and preheat the oven to 350 degrees. Butter and flour taco 7 1/2 by 3 1/2 by 2 1/2-inch loaf pans, dust the interiors with flour, and tap out the excess; set aside. (These are the perfect size pans for these cakes, but they're not always easily found. If you don't have them, use two 8 1/2 by 4 1/2 by 2 1/2-inch loaf pans. Your cakes won't be as tall, but they'll be every bit as flavorful.) Prepare an insulating laver for the cakes by stacking two baking sheets, one on top of the other, or use an insulated (air-cushioned) baking sheet.
Sift the flour and baking ponder together and reserve.
Place the chopped zest and sugar in a large mixing boul and rub them together between your fingers until the sugar is moist, grainy, and very aromatic. Add the eggs and, using a whisk, beat until the eggs are foamy and pale. One by one, add the crème fraîche (or heavy cream), rum, and salt and whisk until the ingredients are incorporated. Using the whisk or a large rubber spatula, gently stir the flour mixture into the batter in four additions; you'll hlave a smooth, thick batter. Finally, fold in the cooled melted butter in hero to three additions.
Immediately pour the batter into the prepared loaf pans, place them on the baking sheet(s), and slip them into the oven. Bake for 55 to 65 minutes, or until the cakes are crowned, split down the center, and golden. A long thin knife inserted into the center of each cake should come out dry and crumb-free. (Check the cakes at the 40-minute mark. If they are browning too quickly, cover them loosely with foil tents for the remainder of the baking period.) Remove the cakes from the oven and turn them out of the pans onto a cooling rack; invert them so they're right side up. Allow the cakes to cool to room temperature before glazing or serving.
If you want to glaze the cakes, place the marmalade in a small saucepan and bring to the boil over low heat (or heat it in the microwave oven); strain the marmalade. Use a broad pastry brush to paint every surface (except the bottoms) of the cakes with a thin coat of glaze. Allow the glaze to dry at room temperature before serving (or wrapping).