Preheat the oven to 325 degrees F and line two cupcake tins with paper liners.

In a large bowl, whisk the flour, cornstarch, baking powder, and salt.

In the bowl of your stand mixer fitted with the paddle attachment, beat the butter and 2 cups sugar until light and fluffy. Scrape down the sides and mix in the 4 whole eggs one at a time. Beat in the vanilla and scrape down the sides again.

Alternating the milk, fold the flour in three batches with a spatula until smooth.

With an ice cream scoop, fill each cupcake tin 2/3 full and bake for 20-25 minutes until toothpick inserted in the center comes out clean.

Cool on a wire rack for 10 minutes before poking a few holes in the top of each and brushing with Irish cream liquor before cooling completely.

Meahwile, make the frosting. Bring water to a simmer in a double boiler. Wisk the 4 egg whites, with the remaining 1 cup sugar in the bowl of the double boiler until the sugar has dissolved, about 3 to 4 minutes. Remove the bowl from the heat and beat with an electric mixer on medium-speed until stiff peaks form, 4 to 7 minutes longer. Beat in the remaining 24 tbs butter, a few pieces at a time, unil smooth. Beat as needed for the mixture to come together (seriously, this can take a minute or two). Whisk the coffee liquor and instant coffee in a small bowl and mix into the frosting. Beat the frosting on medium-high speed until fluffy a few minutes longer.

Ice the cupcakes as desired and add sprinkles if desired!