For blueberries: Put half the blueberries in a pot with sugar. Cook over medium heat for about 5 minutes, stirring occasionally, until berries give off their juices and sugar dissolves. Remove from heat and stir in remaining blueberries, setting aside some whole berries for topping the trifle. Taste for sweetness and add more sugar if needed. Cool to room temperature.
For mixed berries: Set aside some whole berries for topping the trifle. With a potato masher or the back of a spoon, smash remaining berries to break them up slightly. Stir in sugar. Taste for sweetness and add more sugar if needed. Do not oversweeten. Set aside while you make the cream.
For cream: Scrape the seeds from the vanilla bean and discard the rest. Place in a mixing bowl with heavy cream, crème fraîche, sugar, lemon zest and vanilla extract and whisk until soft peaks form.
To assemble the trifle: With a serrated knife, cut sponge cake into quarters and then split each quarter in half horizontally. Spread 1/3 cup of the blueberries in the bottom of a 2 1/2-quart glass bowl. Place a single layer of cake on top, cutting to fit as needed and then a layer of the mashed mixed berries. Cover with a 1/2-inch layer of cream. Cover with more cake. Spread a single layer of blueberries over cake, more cream and then more cake. Repeat layers, alternating between blueberries and mixed berries until the bowl is full. Your top layer should be cream. Sprinkle the reserved whole berries on top. Cover the trifle and refrigerate for at least 4 hours before serving. Spoon into individual bowls to serve.
1 1/4 cups flour
2 1/2 tsp. baking powder
Pinch of kosher salt
5 large eggs, separated
1 1/4 cups sugar
5 tbsp. boiling water
1 tsp. vanilla extract
Preheat oven to 350 degrees. Grease and then line with parchment paper the bottom of an 11 1/2-by-17 1/2-inch baking pan with 1-inch sides.
Sift together flour, baking powder and salt. Set aside.
In the bowl of an electric mixer with the whisk attachment, whip egg yolks and sugar on high speed until thick and pale yellow, about 2 minutes. Reduce to medium low speed, add boiling water and vanilla extract and mix until combined, scrapping the sides of the bowl as necessary. Increase to high speed and again whip until thick, about 2 minutes. Reduce speed to low and stir in dry ingredients.
In a clean bowl of an electric mixer using the whisk attachment, whip egg whites on high speed until soft peaks form. They should be smooth and not clumpy. Fold half of the whipped whites into batter and then fold in remaining whites. Spread batter evenly into prepared pan.
Bake until golden brown and springs back when lightly touched, about 15 minutes.
Makes one 11 1/2-by-17 1/2-inch cake