Preheat the oven to 325º and arrange a shelf in the middle of the oven.

Butter the sides and bottom of a 9” springform pan, then line the bottom with a circle of buttered parchment.

Place the almond paste and granulated sugar in the  bowl of a standing mixer on medium-low speed and cream until the almond paste has been  broken into soft  pieces. This could take some time so please be patient.

Add the softened butter and grated orange zest, cream on medium-high speed until the mixture  somewhat fluffy.

Begin to add the eggs one at a time beating for one minute after each addition, scraping down the bowl midway. Continue until you have incorporated all 4 of the eggs. The batter should be  creamy.

Remove the work bowl from the mixer and begin to add the cake flour & baking powder by hand using a sifter over the work bowl. Gently incorporate the flour using a rubber spatula with deep strokes to be certain that the flour has been combined. Continue to fold in the flour until no traces of white powder remain, the batter will be quite thick.

Turn the batter into the prepared springform pan and spread evenly with an  spatula. Top the cake batter with the sliced almonds.

Place the cake pan into the preheated oven and after 30 minutes quickly open the oven door to test doneness with a wooden toothpick. The cake is done when the toothpick comes out clean, generally about 45 minutes depending upon your oven.

Remove the cake to a wire rack and allow to cool for 30 minutes. At this point run a sharp knife around the sides of the springform pan, being sure to press the knife against the sides of the pan as you go around. Release the spring and carefully remove the sides of the pan. Allow the  Cake to cool completely on the wire rack.

Once the cake is cooled, use two large spatulas to lift it off of the springform pan bottom. Carefully pull of the parchment paper and place the cake on a decorative platter. Dust heavily with confectioner’s sugar if desired.