Method

1. To make the cake. Preheat the oven to 350°F. Grease the bottom of two 9-inch cake pans. Line the bottoms with parchment paper.
2. Sift together the flour, baking soda, salt, and baking powder.
3. Stir together the milk, lemon juice, and vanilla.
4. In the bowl of a stand mixer fitted with the paddle attachment or with a handheld mixer, beat the butter with the sugar on medium-high speed until light and fluffy, 1 to 2 minutes in the stand mixer or 3 to 5 minutes with the handheld. Lower the mixer speed to medium and add the eggs, one at a time, beating for 30 seconds with the stand mixer or 1 minute with the handheld mixer after each addition, periodically scrape down the sides and bottom of the bowl.
5. With the mixer running on low speed, add one-third of the milk mixture. Mix until incorporated and then scrape down the sides of the bowl. Add one-third of the dry ingredients. Mix until combined and scrape down the sides of the bowl. Add the remaining milk and the dry ingredients one third at a time. Mix a few times by hand to be sure all is incorporated, paying particular attention to the bottom of the bowl.
6. Divide the batter between the two prepared pans (24.5 ounces of batter in each) and spread evenly. Bake the cakes on the middle oven rack until a skewer inserted in the middles comes out clean, about 30 minutes.
7. Cool the cakes in their pans.
8. To make the filling. In a small saucepan, combine the blueberries, 3 tablespoons of the sugar and the cassis, if using. Bring to a gentle simmer over medium heat, stirring often, until the sugar is dissolved and the berries have popped open and become juicy, about 5 minutes. Cool to room temperature.
9. In the bowl of a stand mixer, fitted with the whisk attachment, whip the crème fraiche, heavy cream, and the remaining 2 tablespoons of sugar until stiff enough to hold its shape but still smooth.
10. To assemble the cake. Unmold a cake layer by running a small knife around the edges of the pans. Invert onto a cake plate. Carefully peel off the parchment paper.
11. Top with half the whipped crème fraîche and then carefully dollop and spread half the blueberries over the cream, then scatter raspberries and blackberries over the crème fraiche.
12. Unmold and place the second cake layer on top and drizzle with crème de cassis. Spoon more blueberries over the top and drizzle with the juices. Carefully spread the remaining crème fraiche on top and scatter with the remaining blackberries and raspberries allowing a few to fall to the plate.