In a medium saucepan, whisk eggs, egg yolks, and sugar together over medium heat until smooth. Whisk the cornstarch together with a bit of orange juice to create a slurry. Whisk the rest of the juice and the slurry into the egg mixture. Add the lemon juice and food coloring if using.
Continue to whisk over medium heat until thick, 10-15 minutes. Make sure the whisk hits the bottom of the pot to keep the bottom from burning—this is less likely to happen if you use a heavy-bottomed saucepan.
Remove from the heat and slowly whisk in the pieces of butter.
Move the curd to a bowl, press plastic wrap against the top (to prevent a skin from forming) and allow it to chill in the refrigerator for at least 15-20 minutes. It will continue to thicken as it cools.
Prepare the cupcakes:
Line 2 large muffin tins with cupcake liners. Preheat the oven to 350°F and place oven racks in the upper-middle and lower-middle positions.
In a medium-sized bowl, whisk together the flour, cornstarch, baking powder, salt, pumpkin pie spice, and orange zest.
In a small bowl combine the half and half, vanilla extract, and pumpkin puree.
In a stand mixer with the paddle attachment, cream the butter and sugar together on high speed for several minutes. Scrape down the sides, and then add the eggs on low speed, one at a time, scraping down the sides to the bottom of the bowl, until they’re completely incorporated.
On medium-low speed, alternate between adding the dry and wet ingredients over the course of a minute, until the ingredients are just mixed evenly. Give the bowl one more scrape, all the way to the bottom, to make sure there are no patches of flour.
Evenly distribute the batter between the cupcake liners. Bake for 22 minutes, or until the center of one of the cupcakes comes out clean with a toothpick.
Allow the cupcakes to cool completely, remove from the pan, and then use a paring knife to core each cupcake. Before frosting the cupcakes, use a pastry bag (or plastic bag with the corner snipped off) to fill each cupcake with curd.
Prepare the buttercream:
In a stand mixer with the whisk attachment, mix the butter at medium-high speed for 30 seconds.
Add the confectioners sugar and mix on medium-low speed for about 45 seconds. Scrape down the sides and then mix on medium speed until the ingredients are completely combined.
Scrape down the bowl and add the vanilla extract, orange juice, orange zest, pumpkin pie spice and salt. Mix on medium speed to incorporate the ingredients and then turn the mixer to medium-high speed and mix for 4-5 minutes, or until light and fluffy. Scrape down the sides of the bowl once or twice during this process.
There are several steps involved in preparing this recipe, however the curd and cupcakes can be made several days in advance. For best results I recommend making the buttercream frosting the same day you plan on piping the cupcakes.For a simplified recipe, you can omit the orange curd altogether.Don't want to purchase pumpkin pie spice? You can make it from scratch!