Preheat the oven to 350°F. Grease and flour two 9-inch round pans.
Sift together the flour, baking powder and salt. Set aside.
In a large bowl, use an electric mixer to beat the sugar, shortening, eggs and orange zest at high speed for 3 minutes. Beat in the flour mixture a little at a time, alternating with orange juice and milk, beating after each addition until the batter is smooth.
Divide the batter between the prepared pans. Bake for 30 minutes, or until a toothpick inserted into the center comes out clean. Let cool, and then frost with orange buttercream frosting.
PREPARE THE ORANGE BUTTERCREAM:
Use an electric mixer to combine the butter, zest, salt and egg yolk in a medium bowl. Add the sugar alternately with orange juice beating until of spreading or piping consistency.
Notes
Using pasteurized egg yolks in a recipe where the eggs are not cooked reduces risk of salmonella.