Preheat oven to 350°F .
Line two 12 cup muffin tins with paper liners; set aside.
In a medium bowl, sift together the flour, baking powder, and salt.
In a mixing bowl, beat butter for 3 minutes on medium-high speed until the butter is light and creamy in color; stop and scrape the bowl.
Add the sugar and beat until fully incorporated.
Add the eggs one at a time; beating after each addition.
Reduce the mixer speed to low.
In a small bowl, combine the vanilla and buttermilk.
To the mixer alternate adding the dry ingredients alternated with the buttermilk/vanilla mixture and continue to mix just until incorporated.
Scrape the sides of the bowl and mix for 15 seconds longer (don’t over mix)
Spoon the batter into the prepared pans and bake for 18 - 22 minutes or until a toothpick inserted in the center comes out clean.
Remove from oven and let cupcakes cool completely before filling and frosting.
While cupcakes are cooling, make the Popcorn Pastry Cream
Popcorn Pastry Cream Frosting
Place the popped popcorn in a medium sized bowl.
Bring the heavy cream and milk to a boil in a saucepan over medium high heat just until it reaches a boil. Immediately pour it over the popcorn and let the mixture infuse for at least 20 minutes.
Transfer the mixture to a food processor and blend until smooth (about 2 minutes). Pour the mixture through a strainer to get rid of any remaining lumps and kernels.
In a medium bowl, whisk together the egg, egg yolk and cornstarch; set aside.
Combine the popcorn milk mixture and the sugar in a saucepan and bring to a boil, whisking frequently. Once boiling, whisk in the cornstarch/egg mixture. Let it return to a boil until it is thick and smooth, whisking constantly.
Remove from heat and pour into a heat proof bowl. Whisk in the butter, vanilla extract, and sea salt.
Pastry cream can be covered and stored in the refrigerator for up to 3 days.
Decorations – To assemble your cupcakes:
Cut a cone into the center of your cupcakes to hollow out the center, taking care not to cut all the way down through the bottom of the cupcakes.
Fill the hollow center with popcorn pastry cream.
Cut the base off the cone of cake (feel free to eat that piece!) and place the top back on to the cupcake.
Top each cupcake with vanilla icing using an offset spatula.
Cut mini marshmallows in half and then smush them together so they resemble pieces of popcorn. Place marshmallows on top of cupcakes so they cover the entire top of the cupcakes.
Mix one drop of yellow food coloring with a tablespoon of water in a small bowl, then use your paint brush to tab yellow color here and there on marshmallows to enhance the popcorn coloration.