In a saucepan melt the butter for the cake and cook until you see the butter separating; brown bits will start falling to the bottom. Watch it carefully because it can burn quickly and once all the bits are a toasty brown, pour it into a heatproof bowl. Set aside to cool.

Clean and dry the pan then repeat with the butter for the buttercream. Use different bowls or mark them so you know which is for the cake (it will be less) and which is for the buttercream. Once the butter is cool and starts to become creamy and thick, stir it to disperse the brown butter bits. Set in the fridge to chill until ready to use; the consistency of the butter for the cake should be at a soft room temperature.

Preheat the oven to 325 F and grease and flour two round 8 inch pans. If your eggs aren’t at room temperature, set them in a bowl of hot water to warm up.

In the bowl of a stand mixer, add the sugar, salt, vanilla and soft browned butter. Affix the paddle attachment and beat on low speed, then work up to medium. Beat the mix until it’s light and fluffy, the color will be several shades lighter - this can take a few minutes. Add the sour cream and beat for 30 seconds.

With the mixer on, one by one add the eggs. Allow each to mostly incorporate before adding another. Once all the eggs are in, beat well to combine.

Sift in the cake flour and add the baking powder. Turn the mixer on its lowest setting (so flour doesn’t fly out!) and while it’s slowly beating, pour in the milk. Beat until the batter is well combined, stop the mixer to scrape down as needed.

Divide the cake batter between the two prepared cake pans. Bake until a cake tester comes out clean, about 30 minutes. Let cool briefly in the pan then transfer to a wire rack to cool fully.

Chop up the chocolate and set in a small bowl. Heat the 100g of heavy cream until it’s steaming and pour over the chocolate. Stir briefly then leave it for a minute, stir until smooth. Let cool until thickened before proceeding.

Separately, beat the remaining heavy cream with the vanilla and salt to medium peaks; when the mixer is leaving ribbons in the cream as it moves. Add half of the cooled, thick ganache to the bowl and briefly mix. Add the remaining ganache and mix to combine. If the mousse is starting to get overly thick and grainy, add a bit more heavy cream and stir until it’s airy.

Make the Brown Butter Swiss Meringue Buttercream

The temperature of the butter for the buttercream should be on the cooler side, but not rock hard. If you find you need to warm it and it becomes soft, we’ll troubleshoot this afterwards. But don’t use melted or overly soft butter! Chill it to firm it up.

Fill a pot about 2 inches high with hot water and set it over medium low heat. Set a bowl over the pot (this will act as a double boiler) and bring the water to a simmer. Pour the sugar and egg whites into the bowl and immediately whisk well.

Keep stirring the mix as it heats. If you’re using a rubber spatula you can scrape the bottom of the bowl and if you hear/feel scratching that usually means the sugar has yet to dissolve. If you can lift the spatula and pinch the mixture and detect no sugar granules, it’s done heating. Pour it into the bowl of the stand mixer (the bowl should be very clean and dry) and affix the whisk attachment. Add the cream of tartar and begin whisking on low, then work up to medium.

Whisk until you have stiff peaks, where you can lift the whisk upside down and it doesn’t droop. Add the vanilla and salt and whisk briefly then, with the mixer on low, add the butter, about a tablespoon at a time. The meringue will turn into soup and depending on the temperature of the butter will either whip up into a creamy buttercream or, if the butter was on the warm side, stay a loose soup. If it is not thickening this is because the butter is too warm; set the bowl in the freezer for 10-15 minutes then try again.

Slice off the domes from the cake. Place one of the cakes sliced side up and spread a thick layer of mousse over it (if you pipe a border of buttercream around the bottom layer it helps it from slipping out). Set the other cake sliced side down and spread the top and sides with the buttercream. Top with chocolate curls or shavings or berries, if you like.