Preheat the oven to 325 degrees. Butter and flour a 9inch springform pan and set it aside.

Break up the almond paste and place it in the container of a food processor. Process for a few seconds until it is grainy, like the texture of cracked wheat. Set aside.

In a large bowl, using an electric mixer, beat the butter and sugar together until very fluffy, at least 3 minutes. Add the almond paste, orange liqueur, and orange zest. Beat until perfectly blended, another 2 minutes or so.

One by one beat in the eggs until the mixture is very smooth and fluffy, about 2 minutes. Sprinkle on the flour and baking powder and beat just until combined, 30 seconds or so.

Scrape the batter into the prepared pan. Bake 55 minutes, or until a knife inserted in the center of the cake comes out clean, and the sides of the cake have begun to shrink away from the pan. Cool on a wire rack for 10 minutes. (The cake might sink a little in the center.)

Remove the outer ring of the pan. Place a plate over the cake and invert it. Remove the bottom of the pan. Invert again onto a wire rack and cool the cake completely. Serve dusted with confectioners sugar.