Preheat the oven to 350°F. Grease a 9'' springform pan with baking spray or neutral oil and line with parchment paper.

In a large bowl, combine the sugar, buttermilk, vegetable oil, eggs, and vanilla extract. Whisk until smooth.

Add the flour, malted milk powder, cocoa powder, baking soda, and Kosher salt. Whisk just until the batter is combined (don't over-mix!).

Pour the batter into the prepared springform pan. Bake for 30-35 minutes until an inserted toothpick has a few moist crumbs on it. Let the cake cool in the pan for about 10 minutes. Then, use a butter knife to loosen the edges of the cake from the pan. Flip onto a wire rack and cook completely.

In a large bowl, beat the butter and peanut butter together until the mixture is smooth.

Add the powdered sugar and malted milk powder. Mix until the frosting is thick. Then, add the vanilla extract, milk, and a pinch of salt. Beat the mixture until it's very fluffy and whipped, 3-4 minutes. Add more milk to thin the buttercream, if necessary.

Once the cake is cooled, you're ready to assemble! If the cake has a dome, slice it off to level the cake (totally optional, this is only for aesthetics).

Dollop the frosting on top of the cake and use an offset spatula or butter knife to frost the top of the cake. Use the tip of the offset spatula to create swirls.

Finish the cake with a sprinkle of flaky salt. Or, decorate as desired. Slice and enjoy!