Place 2 cups butter into a large, shallow, light colored saucepan. Cook over med-low heat, stirring frequently until the butter starts to brown and smell nutty.**
Immediately transfer to bowl or large measuring cup. Be sure to scrape in the brown bits. Measure out two separate 3/4 cup portions into separate containers for the cake and frosting respectively. Let cool to room temperature and solidify.***
Brown Butter Cake:
Preheat oven to 350F. Grease and flour two 8" cake rounds and line with parchment.
In a medium bowl, whisk flour, baking powder, and salt until well combined. Set aside.
Using a stand mixer fitted with a paddle attachment, cream browned butter and sugars on med-high until pale and fluffy (approx 3mins). Reduce speed and add eggs one at a time fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture and milk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition.
Bake for 45mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack and cool completely.
Brown Butter Swiss Meringue Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.****
Place bowl over a double boiler on the stove and whisk constantly until the mixture is hot and no longer grainy to the touch (approx. 3mins). Or registers 160F on a candy thermometer.
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment. Slowly add all of the regular butter and then the browned butter 1 Tbsp at a time and mix until smooth.*****
Add vanilla and whip until smooth.
Assembly:
Place one layer of cake on a cake stand or serving plate. Top with 1 cup buttercream and spread evenly.
Place second cake layer on top and apply a thin coat of frosting all over to crumb coat. Chill for 20mins.
Frost & smooth the top and sides of the cake. Pipe rosettes on top using a 1M tip if desired.
Notes
*2 cups butter is more than you will need, but I always like to make more than less as some evaporates. Any extra can be used as a sauce for meant or vegetables. ** Be careful not to burn it. It will go from brown to black very quickly. *** Once the butter is at room temperature, you can place it in the fridge to speed up the solidifying process. Be sure to bring it to room temperature before using in the cake and frosting recipes. **** Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk is completely grease free or your meringue won't stiffen. ***** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth.NOTE: The Brown Butter Swiss Meringue buttercream recipe has been revised to add in more regular butter. The original recipe was browning 3 cups of butter and using 3/4 of that for the cake and 1 1/2 cups for the frosting. The frosting was: 6 whites 1 1/2 cups sugar 1 1/2 cups brown butter 1 tsp vanilla