Instructions

1. Heat the oven to 350°F. Coat 2 (9-inch) round cake pans with butter or cooking spray. Coat each pan with a light dusting of all-purpose flour, tapping out the excess. You can also line the bottom of each pan with a parchment paper round for extra insurance, if you'd like.
2. Beat 1 stick room temperature unsalted butter, 1 cup granulated sugar, 1/2 cup packed light brown sugar, and 1 teaspoon kosher salt together in a stand mixer with a paddle attachment on medium-high speed until lightened in color, about 3 minutes. Beat in 3 large eggs one at a time, waiting until each is fully incorporated before adding in the next.
3. Add 2 1/4 cups all-purpose flour and sprinkle evenly with 3 1/2 teaspoons baking powder. Beat on the lowest speed until just combined. Add 1 1/4 cups whole milk and 1 teaspoon vanilla extract and beat on low speed for 30 seconds. Increase the speed to high and beat until combined and it gets a bit lighter, 2 to 3 minutes. Scrape down the sides of the bowl with a flexible spatula as needed.
4. Divide the batter between the cake pans (about 20 ounces each). Bake until the tops spring back slightly when pressed and the cakes pull away from the sides of the pans, 23 to 30 minutes.
5. Let the cakes cool in the pans for 15 minutes. Flip the cakes out of the pans onto a wire rack, remove the parchment, and let cool completely, about 1 hour. Meanwhile, wash and dry the stand mixer bowl. Make the frosting.
6. Whisk 2 1/2 cups of the granulated sugar, 1 cup heavy cream, and 1 tablespoon all-purpose flour together in a medium saucepan until smooth. Bring to a boil over medium heat, whisking often. Turn off the heat.
7. Place the remaining 1/3 cup granulated sugar in a wide, deep, large heavy-bottomed pot or enameled Dutch oven. Cook over medium-high heat, stirring constantly, until the sugar turns into a golden-brown caramel, about 2 minutes. The sugar will clump at first, but continue to stir and it will eventually melt and begin to brown.
8. Remove the pot from the heat. While whisking constantly, slowly pour in the cream mixture (careful, it will bubble up!). Whisk until smooth.
9. Return the pan to medium-high heat and cook, whisking occasionally, until the mixture registers about 240˚F on a candy or instant-read thermometer, 1 to 2 minutes.
10. Remove the pot from the heat. Stir in 1 stick of the room temperature unsalted butter and 1 teaspoon vanilla extract until the butter is fully incorporated. Let cool, stirring occasionally, until the mixture is 110°F or cooler, about 1 1/2 hours. If you're in a hurry, fill a clean sink halfway with cold water and ice. Set the pot in the sink and stir, being careful to avoid any water getting into the pan, until the mixture is the right temperature.
11. Transfer the mixture to the stand mixer. Beat with the whisk attachment on high speed until it crystallizes and lightens in color, about 10 minutes. Add the remaining 1 stick room temperature unsalted butter and continue to beat on high speed until the butter is completely incorporated and the frosting has a texture like buttercream, about 5 minutes more.
12. If the cakes are domed, use a long serrated knife to trim off the dome so they are level. Set 1 cake layer on a cake or serving plate. Transfer 1 cup of the frosting onto the cake layer and use an offset spatula to spread it out evenly to the edges. Top with the second cake layer trimmed-side down.
13. Transfer the remaining frosting on top of the cake. Spread the frosting evenly on top of the cake and onto the sides.
14. Let sit for at least 3 hours before slicing and serving. If you’d like a fudgier texture in the frosting, refrigerate before serving. This cake is even better the day after it is made.