Preheat the oven to 350° F. Prepare two 12-cup standard cupcake pans: grease and flour the cupcake wells or use paper liners.
In a large bowl, whisk together the flour, cornstarch, baking powder and salt. Set aside.
In the bowl of a stand mixer beat together the 1 cup plus two tablespoons of butter and the granulated sugar for 5 minutes: the mixture should be fluffy and pale yellow in color.
Add in the two whole eggs one at a time, mixing after each addition. Slowly add in the egg yolks while the mixer is running on low. Mix in the vanilla extract and the orange zest.
Add in about ⅓ of the flour mixture and mix the batter on low speed. Add in half of the milk and mix it into the batter. Repeat with another third of the flour mixture, the rest of the milk and then the last of the flour until the ingredients are incorporated and the batter is smooth.
Fill each cupcake well about ⅔ full with batter (2 ounces in each well.) Place the pans in the oven and bake for 20 minutes until a toothpick inserted into the centers comes out clean or with only a few crumbs.
Turn the cupcakes out onto a cooling rack and allow them to cool completely.
Prepare the buttercream:
Beat together the ¾ cup of butter and the ¼ cup of vegetable shortening in the bowl of a stand mixer.
Beat in 1 cup of the confectioner's sugar, followed by a little of the milk. Continue alternating the sugar and milk until all of the sugar has been mixed in. (You may not need all of the milk.)
Add in the vanilla and beat the frosting until it is light and fluffy.
Fill the cupcakes:
Cut out the centers of the cupcakes: use a sharp knife to cut out the center of the cupcakes, by inserting the tip of a knife ⅔ of the way into the top of the cupcake. Cut out a one-inch cylinder of cake from each cupcake and set them aside.
Scrape out the well inside the cupcake just a bit so that there is room for the mango jam. Drop 1 teaspoon of Mango Jam into the well.
Cut off most of the saved cake cylinders to just leave the tops: these will go back on the cupcakes. Place these cake caps over the jam-filled holes and press them in place gently to make the cap flush with the top.
Frost the cupcakes:
Spread a thin layer of frosting on each cupcake as a crumb coating.
Place the rest of the frosting in a piping bag with a 2D tip. Starting in the center of the cupcakes make a small mound of frosting and then pipe frosting out and around about two times to cover the top of the cupcakes. (Alternatively if you'd rather not pipe, just use a knife to spread frosting on each cupcake. Make the frosting thick enough to hold up slices of orange.)
Finish with orange slices:
Wash the oranges well, dry them, and slice them into ½-inch thick slices. Cut the slices into semi-circles.
Blot the excess juice off of the slices with a paper towel.
Place one semi-circle slice onto each cupcake, cut side down. Push the slice in gently so that it stands up in the frosting.
The cupcakes can be stored in a cupcake holder or covered in plastic wrap, and stored in the fridge for up to three days.
Makes about 23 cupcakes.
Notes
Read the recipe through before starting. The ingredients are divided for each component of these cupcakes, and some ingredients are listed more than once.
The cake recipe is slightly adapted from the recipe in The Baker's Manual by Amendola/Rees. The buttercream is slightly adapted from the recipe in the King Arthur Baker's Companion.