1. First, brown your butter. Melt butter in a saucepan and continue to cook on medium-low, watching carefully for bubbles to subside and the bottom of the liquid to begin browning. Once it is taking on a golden brown color, remove from heat. It should smell nutty and have little brown bits in it.
2. Pour in a small bowl and allow to cool back to room temperature. It should be soft and spreadable.
3. Preheat oven to 350 F. Spray two 6" cake pans and line bottom with parchment paper.
4. In a small bowl, combine milk, egg whites, and vanilla.
5. In the bowl of a stand mixer fitted with the paddle attachment, combine dry ingredients until thoroughly combined. Add soft brown butter, blending until mixture looks like breadcrumbs.
6. Add all but 1/4 cup of milk mix and beat on medium speed until thick and creamy, 2-3 minutes. With mixer on low, gradually pour in remaining milk.
7. Divide batter evenly between the prepared pans and use a spatula to even out.
8. Bake until golden brown on top and sides are beginning to pull away from the pan, about 25-30 minutes. Allow to cool completely.
1. Allow berries & rhubarb to defrost slightly.
2. Place in a stainless steel pot with the sugar and cook until fully broken down and quite thick, stirring and mashing with a wooden spoon all the while. This should take about 10-15 minutes. If using fresh fruit, it may take longer to thicken.
3. Pour compote in a bowl and allow to cool completely.
Whisk cream, sugar, and vanilla until soft peaks form.
1. Slice each cake in half horizontally to make 4 layers.
2. Spread first layer with whipped cream, followed by a layer of compote. Repeat on next 2 layers.
3. Top with final cake.
4. Apply a thin layer of whipped cream around entire cake for a crumb coating.
5. Chill at least 20 minutes; ice with remaining whip.
Keep cake in fridge until ready to serve. This cake is great chilled or at room temperature.