Combine the sugar and the flour in a heavy saucepan. Whisk to mix, add the orange juice and egg yolks, whisk quickly to combine. Put the saucepan over medium-high heat and cook, whisking constantly, until the mixture boils. (about 3 minutes) Cook another minute, stirring constantly, the mixture will thicken and become less cloudy. Be sure to let it boil for one minute so that the filling will thicken enough to support the top layer of the cake. Remove from heat and stir in the butter. Transfer to a bowl and cover with plastic wrap, laying the plastic wrap directly on the filling when covering the bowl.

Chill thoroughly before using.

Heat the oven to 350 degrees. Sift together flour, baking powder and salt, set aside. Grease and flour two 8″ round cake pans (you can use 9″ round, the layers will just be thinner) 

In your mixer, cream together butter, shortening, and zest. Gradually stir in the sugar and eggs, creaming together, and scraping down the sides of the bowl.

Add the sifted dry ingredients alternately with the orange juice to the creamed mixture, beating well on low speed after each addition. Pour equal amounts of batter into the two prepared cake pans. Tap the pans on the counter a couple of times to remove any air bubbles and to even out the batter. Bake until a toothpick in the middle comes out clean, about 28 minutes. Cool the layers in the pans for about 10 minutes and then loosen the layers from the pans and transfer to a rack to cool completely.

Cream the butter in the mixer. Add the sugar and salt and combine thoroughly. Add the orange juice and zest and mix until light and creamy. Make sure to scrape the sides of the bowl down. Refrigerate if not using right away.

(if you refrigerate, be prepared to place frosting back under the mixer to fluff it back up when you are ready)