Preheat oven and prep pans: To make the cake, preheat the oven to 350°F and arrange a rack in the center. Grease one 8-inch round cake pan (for the top layer) and one 8-inch springform pan (for the bottom layer) with shortening and lightly dust each with flour.,

Sift flour, baking powder, and salt: Over a large bowl, sift together the cake flour, baking powder, and salt. Set aside.,

Beat shortening, add sugar, eggs, and flour mixture: In a large bowl and using an electric hand mixer, or in a stand mixer fitted with the paddle attachment, beat the shortening on medium speed while gradually adding the granulated sugar. Beat until light and fluffy, about 2 minutes. Add the eggs, one at a time, beating well after each addition. Add the sifted flour mixture and continue to beat until thoroughly combined and the batter is smooth.,

Add milk, orange juice, and zest; transfer to pans and bake: Continue mixing, adding the milk, orange juice, and orange zest. Mix well until the batter is smooth, another 1 to 2 minutes. Divide the batter evenly between the prepared pans. Bake on the center rack until golden brown, about 35 minutes.,

Cool cakes: Place the pans on a cooling rack. After 5 to 10 minutes, gently flip the 8-inch cake pan (top layer) and return the cake to the rack to cool. Leave the 8-inch springform cake (bottom layer) in its pan, but allow it to cool for 10 minutes.,

Turn on broiler: Meanwhile, turn the broiler to high.,

Arrange marshmallows on cake layer and broil: Arrange the marshmallows evenly on top of the bottom cake layer (still in the spring-form pan) and place it under the broiler until the marshmallows are soft and lightly browned, 1 to 2 minutes. Remove the pan and allow the cake to cool completely.,

Make the orange glaze: To make the orange glaze, in a large bowl and using the hand mixer or the stand mixer fitted with the paddle attachment, beat the butter on medium speed until light and fluffy, a minute or so. Add the orange zest and salt and mix to combine. Add half of the powdered sugar and half of the orange juice and continue to blend until thoroughly combined. Add the remaining powdered sugar and orange juice and continue blending until the glaze is smooth and thin enough to spoon over the cake.,

Assemble cake: Remove the bottom cake layer from the springform pan and transfer it to a cake plate. Spoon about half of the orange glaze over the toasted marshmallows. The glaze should run down the sides of cake. Top with the second cake layer (rounded top facing up) and finish by spooning the remaining orange glaze over the top. Slice and serve.