Heat the oven to 350 degrees F. Beat the sugar and butter together until creamy. Beat in the eggs. Sift the flour and baking powder over the mixture and fold them in lightly.
Grate the zest of the orange and squeeze out the juice. Stir zest, juice, and Cointreau into the batter. Then grease a 5-cup bread pan and spoon in the batter. Knock the pan against the countertop to settle the contents. Bake for about 50 minutes, until the cake is firm but bouncy to the touch and the blade of a knife inserted into it comes out clean. Leave it to cool for 10 to 15 minutes, then unmold onto a dish.
Meanwhile, prepare the glazing. In a small saucepan, gently warm the orange juice and Cointreau. Stir in the confectioners' sugar.
Using a pastry brush, paint the glaze over the cake--some of the glaze will seep in, the rest will form a light crust. Serve cold.
Recipe courtesy of CUISINE GRAND-MERE (Time-Life Books, 2001) by Marie-Pierre Moine