In a medium saucepan, add in the sugar and cornstarch. Whisk to incorporate the cornstarch into the sugar. This will help prevent lumps when heating.
Whisk in the lemon juice and the fresh blueberries. Mash the blueberries in the saucepan once the heat softens them a bit.
Bring to a boil over medium heat and let boil for 4 minutes until thick. Be sure to stir constantly. The filling will become clear rather than cloudy and blue/purple.
Place in a bowl and cover. Chill in the fridge.
Preheat oven to 350 degrees. Place cupcake papers in the cupcake pan.
In a mixing bowl, add in the butter, sugar and oil. Mix until light and fluffy. This will take about 2 minutes.
Add in the room temperature eggs one at a time and mix until combined. Once they are mixed in, turn the hand mixer to high and beat for another 3 minutes until the mixture becomes thick and very pale. This is crucial otherwise the cake will fall.
Measure out the milk, limoncello, lemon extract and zest. Set aside.
Measure out the cake flour, baking powder and baking soda.
Add a third of the flour mixture into the egg mixture. Beat just until combined.
Add half of the milk and beat until just combined.
Repeat adding another third of the flour then the rest of the milk. Finish with the last bit of flour.
Scoop out the batter and place in the cupcake liners. Only fill about ⅔ full.
Place in the oven and bake for 17 minutes or until done. Place a toothpick in the center of the cupcakes and if it comes out clean or with moist crumbs it's done. If there's still batter, give it another minute or two.
Remove from the oven and let cool.
Once completely cooled, cut out holes in the center of the cupcakes. Only go about three-quarter of the way down the cupcake for the holes.
Place the chilled blueberry filling in each of the cupcakes. Keep the filling flush with the cupcakes and not peaking out of the top.
If you want to get real boozy, add a ½ teaspoon to 1 teaspoon of limoncello to the top of the cupcake. Don't oversaturate it, otherwise you'll have a soggy cupcake.
In a large mixing bowl, add in the butter. Mix until the butter is creamy.
Add in the lemon juice, limoncello, and powdered sugar. Mix on low until the powdered sugar is almost completely incorporated.
Turn mixer on high and beat for about 1 to 2 minutes until light and fluffy.
Place a 1M piping tip into a disposable piping bag. Line one crease of the piping back with the yellow gel food color.
Fill bag with the frosting. Pipe nice tall swirls on each cupcake.
Garnish with a lemon zest curl and fresh blueberry.