Mini Vanilla Cake with Brown Butter Frosting

Instructions

1. Preheat the oven to 350°F/180°C. Grease two 6-inch cake pans and line the bottom of the pans with parchment paper.
2. In a small bowl, whisk together the flour, baking powder, baking soda, and salt, and set this flour mixture aside.
3. In a bowl of a stand mixer fitted with a paddle attachment or in a medium bowl and electric mixer, beat room-temperature butter and oil on medium speed until it's creamy and combined. Add in the granulated sugar and beat on high speed for around 3-4 minutes until it's very light in color and creamy.
4. Add in the vanilla and eggs, one at a time, and beat in after each addition.
5. In a small bowl, stir together the milk and vinegar.
6. Add half the dry ingredients to the butter mixture, stir them in, then add half the milk. Use a rubber spatula to stir it in well until well combined. Repeat with the rest of the dry ingredients and milk mixture.
7. Divide the cake batter evenly amongst the cake pans and smooth down the tops of the batter with a small offset spatula.
8. Bake the cakes for around 26-28 minutes until the tops spring back when touched and a skewer inserted into the middle of the cakes comes out clean.
9. Remove the cakes from the oven and let them cool for 10 minutes in their pans, then turn them onto a wire cooling rack and allow them to cool completely.

The brown butter for the buttercream

1. While the cakes are baking, prepare the brown butter for the frosting, as this needs time to cool. Over medium heat, melt 75g (1/3 cup) of the butter. Avoid using a dark pan, as you will struggle to see the caramelization of the butter and run the risk of burning it.
2. Once you have melted butter, let it cook for around 3 minutes, occasionally stirring until the milk solids in the butter have caramelized into a golden amber color and the butter smells nutty.
3. Pour the browned butter into a flat dish. Ensure you get all the browned bits, as that's where the flavor is. Refrigerate it until it has solidified (but not so cold that it's rock-hard).

Swiss Meringue Buttercream

1. Bring a pot of water (with 1 inch of water) to a simmer. 
2. Add egg whites and granulated sugar to the stand mixer bowl. Balance the bowl over this simmering water bath. Whisk continuously as the egg whites warm up and keep whisking until all the sugar has dissolved or the temperature reads 160°F (71°C) on an instant thermometer. Don’t stop stirring, or some egg whites may overcook on the edges of the bowl. 
3. Once the sugar has dissolved, place the bowl in your stand mixer fitted with the whisk attachment. Beat on medium-high speed until a meringue with stiff peaks forms and the meringue has cooled down to room temperature. When you touch the sides of the bowl, they shouldn't feel warm. 
4. With the mixer on medium-high speed, feed in the solidified brown butter, followed by the rest of the butter, a cube at a time. Allow the butter to incorporate fully before adding the next cube. At this point, the frosting can suddenly appear split or curdled and look like cottage cheese. Just keep beating the mixture until it becomes a thick and cohesive buttercream. This can take a lot of beating, so don't give up if it doesn't seem to come together - it will. Keep beating it until it no longer looks greasy.
5. Once the mixture is thick and cohesive, beat in the vanilla and salt until thick and fluffy.
6. Switch to the paddle attachment and beat on low speed for 10 minutes to remove some larger air bubbles in the buttercream.

Frosting the cake

1. Only frost the cake once the cake layers are completely cooled. You can speed up the cooling process by placing the cakes in the fridge or freezer.
2. Place one cake layer on a cake turntable or cake stand. Scoop on some frosting and use a small offset spatula to spread the frosting on top of the cake layer. Add on the second cake layer. Placing the top layer upside down will give a nice, smooth top to work with.
3. Add a few scoops of frosting on top, spread it out with the spatula, and add a thin coat of frosting around the cake. Use a bench scraper to smooth it out, then chill the cake for around 20 minutes in the fridge until the frosting has firmed up.
4. Add a thicker layer of frosting to the cake and use a bench scraper to smooth it out. Then, you can use a serrated bench scraper to make patterns on the side of the cake if you wish.
5. Leftover cake can be stored in an airtight container in the fridge for up to 4 days.