Make the cereal milk: Separate the marshmallows and cereal from the Lucky Charms™ and set the marshmallows aside for assembling the cupcakes.
2. Step
Add the cereal and milk to a large liquid measuring cup. Let the cereal soak for 20 minutes, stirring occasionally. Drain the cereal milk through a fine-mesh sieve into a clean liquid measuring cup. Discard the cereal and set the cereal milk aside.
3. Step
Make the cupcakes: Preheat the oven to 350°F (180°C). Line a 12-cup muffin tin with paper liners.
4. Step
In a medium bowl, whisk together the flour, baking powder, and salt.
5. Step
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar together on medium speed for about 3 minutes, or until light and fluffy.
6. Step
Add the eggs, 1 at a time, beating between each addition until incorporated. Add the sour cream and vanilla, and beat until combined.
7. Step
With the mixer running on low speed, alternate additions of the flour mixture and ¾ cup of the cereal milk until incorporated. Do not overmix.
8. Step
Fill each cupcake liner with the batter to about ⅔ of the way full. Bake for 18–20 minutes, or until the tops of the cupcakes feel springy to the touch. Remove from the oven and let cool completely.
9. Step
While the cupcakes are cooling, make the buttercream: Add the flour and remaining ¾ cup cereal milk to a small saucepan and whisk until all of the flour is incorporated. Place the saucepan over medium heat. Cook, whisking continuously, for about 2 minutes, or until thickened. Immediately remove the pot from the heat and add the sugar and salt. Stir until the sugar has completely dissolved; the mixture will thin out after the sugar is added.
10. Step
Transfer the mixture into a shallow dish and cover with plastic wrap, pressing directly against the surface to prevent a skin from forming. Transfer to the refrigerator for about 1 hour, until cool, but not fully chilled.
11. Step
Once the flour mixture has cooled, beat the butter in the bowl of a stand mixer fitted with the whisk attachment until light and fluffy, about 3 minutes. Slowly add the flour mixture, 1 spoonful at a time, until completely incorporated. Add the vanilla, then increase the mixer speed to medium-high and whip until the frosting is light and fluffy. (It should hold its shape on the whisk.) If the frosting is too runny, chill the bowl for 30 minutes before whipping again. If it is too thick and chunky, let sit at room temperature for 30 minutes before whipping again. Transfer the frosting to a piping bag fitted with a large open star tip.
12. Step
Assemble the cupcakes: Cut out the center of each cupcake and fill each with 5–6 Lucky Charms™ Marshmallows. Pipe the buttercream in a swirl on top of each cupcake. Crush some of the remaining marshmallows between your fingertips and sprinkle on top of the cupcakes. Use a small, food-safe brush to apply small bits of gold leaf to the tops of the cupcakes, then place a whole marshmallow atop each cupcake.
13. Step
Serve the cupcakes at room temperature. Leftover cupcakes will keep in an airtight container at room temperature for up to 3 days.
