1.Combine the 2 tablespoons sugar, 1 teaspoon brown sugar, 1/6 cup cake flour, 1/8 teaspoon baking powder, 1/8 teaspoon salt and 1/2 tablespoon rainbow sprinkles in the bowl of a stand mixer fitted with the paddle attachment and mix on low speed until well combined.
2.Preheat the oven to 300 degrees. Combine 3/4 teaspoon oil and 3/4 teaspoon vanilla in a small container and stream them into the mixer while paddling on low. Scrape the bowl then paddle on low again. Give it one last mix and pour the crumb out onto a quarter sheet pan lined with parchment. Use your fingertips to spread the crumb out evenly. The largest crumbs should be no larger than a quarter.
3.Bake for 20 minutes, then check to make sure they are only browning very slightly. Crumbs should contract just a bit to form individual boulder shapes when they’re done. Let the crumb cool completely before using. Stored in an airtight container, the crumb will keep fresh for 1 week at room temperature or 1 month in the fridge or freezer.
4.Preheat the oven to 375 degrees and line 1, 12 cupcake pan with liners. Combine the 2 tablespoons butter, 2 1/2 tablespoons shortening, 1/2 cup plus 2 tablespoons sugar, 1 1/2 tablespoon brown sugar, in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 1-2 minutes, just until it all comes together.
5.Scrape, and add 1 1/2 eggs one at a time, making sure each is fully incorporated before adding the next. Mix on high for 2-3 minutes. Scrape. On low speed, stream in 2 1/3 tablespoon vegetable oil and mix until the mixture is emulsified. Then stream in 7 1/4 tablespoons buttermilk, just a little at a time to avoid breaking the emulsion. Spend a good 1-2 minutes slowly streaming in the buttermilk. When all buttermilk is streamed in, add 1 teaspoon vanilla. Increase the speed to medium-high and paddle for 4-6 minutes until the mixture is practically white, twice the size of the original fluffy butter-and-sugar mixture, and completely homogenous.
6.Combine 2 cups cake flour, 3/4 teaspoon baking powder, and 1/4 teaspoon plus 1/8 teaspoon salt in a medium bowl. On the lowest mixer speed, add the dry ingredients into the wet all at once and mix for 45-60 seconds, just until the batter comes together. Scrape. Remove the paddle attachment and add in 2 tablespoons rainbow sprinkles, and gently fold them into the batter with a spatula.
7.Evenly spread the batter amongst the lined cupcake pans. You should have just enough to fill each liner just under 1/2 way full, about 2-3 Tablespoons. Sprinkle remaining 1 tablespoon sprinkles evenly over tops of batter.
8.Bake cupcakes for 15-20 minutes, rotating after 7 minutes. Check cupcakes at 15 minutes, they should not jiggle, and slightly bounce back when pressing on the tops of their domes with your fingers. Bake for an addition 2-5 minutes if needed.
9.Combine 2 tablespoons milk and 1/2 teaspoon vanilla extract together, give it a mix and place in fridge until needed. Remove the cake from the oven and allow to cool for 10 minutes in pan, then transfer cupcakes to wire rack to finish cooling to room temperature.
10.Place 8 tablespoons butter in a large mixing bowl. Beat with an electric mixer on medium speed for 1-2 minutes until butter is smooth and creamy. Add 1 teaspoon vanilla and 1/2 cup powdered sugar. Beat on a low speed to combine, then turn up the speed to medium and mix for 30 seconds. Next, add 1 cup powdered sugar, along with 1 tablespoon sour cream.
11.Beat on medium speed until combined. Scrape down the sides of the bowl. Add final 1/2 cup powdered sugar, 1 tablespoon milk and 1/16 teaspoon baking powder. Beat on medium speed until frosting is fluffy and creamy.
12.Dip a pastry brush into your milk soak and gently splash a little onto each of your cupcakes. With a spatula, frost cupcakes. Garnish the top of each cupcake with your larger crumb pieces or sprinkle smaller pieces on top. You may also garnish with more rainbow sprinkles if you’d like. Enjoy cupcakes immediately or store in an airtight container in the fridge for up to 5 days.