Finely grind the almonds in a food processor.
With an electric mixer, beat the egg yolks with the sugar to a smooth pale cream. Beat in the zests and almond extract. Add the ground almonds and mix very well.
With clean beaters, beat the egg whites in a large bowl until stiff peaks form. Fold them into the egg and almond mixture (the mixture is thick, so you will need to turn it over quite a bit into the egg whites).
Grease an 11-inch springform pan, preferably nonstick, with butter and dust it with flour. Pour in the cake batter, and bake in a preheated 350°F oven for 40 minutes, or until it feels firm to the touch. Let cool before turning out.
Just before serving, dust the top of the cake with confectioners' sugar. Or, if you like, cut a St. James cross out of paper. Place it in the middle of the cake, and dust the cake with confectioners' sugar, then remove the paper.