Instructions

1. Make the cake: Heat the oven to 350°F. Line a 9×9 square pan with parchment paper.
2. Combine the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 2 to 3 minutes, until light and fluffy. Scrape down the sides of the bowl halfway through this process, and again at the end of it.
3. Add the eggs, one at a time, beating on medium-high for 1 minute after each addition. After you add the last egg, scrape down the sides of the bowl, then beat on high for 4 more minutes. Scrape down the sides of the bowl again.
4. Combine the buttermilk and oil, and with the mixer on medium speed, stream them into the batter very slowly. It should take you approximately 3 minutes to add these liquids.
5. Scrape down the sides of the bowl, increase the mixer speed to medium-high, and paddle for an additional 2 to 3 minutes, until the mixture is practically white, twice the size of your original fluffy butter-and-sugar mixture, and completely homogeneous. Don’t rush the process. You’re basically forcing too much liquid into an already fatty mixture that doesn’t want to make room for that liquid. There should be no streaks of fat or liquid. Stop the mixer and scrape down the sides of the bowl.
6. Whisk together the cake flour, corn powder, cornstarch, baking powder, and salt in a medium bowl.
7. With the mixer on very low speed, slowly add the dry mixture and mix for 45 to 60 seconds, just until your batter comes together. Scrape down the sides of the bowl, then mix for an additional 45 seconds to make sure no lumps of flour or powder get left behind.
8. Spread the cake batter evenly into the pan.
9. Bake for 30 to 35 minutes, rotating the pan front to back halfway through baking.
10. The cake will rise and puff, doubling in size, but will remain slightly buttery and dense.
11. At 30 minutes, gently poke the edge of each cake with your finger: The cakes should bounce back slightly and the centers should no longer be jiggly. If they don’t pass these tests, leave the cakes in the oven for an additional 3 to 5 minutes.
12. Let the cakes cool in the pan.
13. Make the crumb: Heat the oven to 250°F. Line a sheet pan with parchment paper or a silicone baking mat.
14. Combine the milk powder, flour, cornstarch, blueberry powder, sugar, and salt in a medium bowl. Toss to mix. Add the melted butter and toss, using a spatula, until the mixture starts to come together and form small clusters.
15. Spread the clusters on the lined sheet pan and bake for 20 to 25 minutes. The crumbs should be sandy, but still clumped together, just starting to brown around the edges. Cool the crumbs completely. The crumbs will keep in an airtight container in the fridge or freezer for up to 1 month.
16. Make the frosting: Combine the butter, confectioners’ sugar, and salt in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 6 to 8 minutes, scraping the bowl down once to make sure there are no butter lumps, until the mixture is smooth and fluffy.
17. Scrape down the sides of the bowl, then, with the mixer on low, stream in the sour cream and continue creaming until the puree is fully emulsified and the frosting is smooth and fluffy again.
18. Increase the speed of the mixer to medium-high and mix until the entire mass is bright white, silky, and shiny. It should hold medium soft peaks, like whipped cream.
19. Use the frosting immediately to frost the cooled cake.
20. Sprinkle the blueberry crumb on top. Refrigerate until ready to serve!