Berry Creme fraiche cake

Poppy seed cake
2 1/2 cups flour
1 teaspoon baking soda
1/2 teaspoon salt
1 teaspoon baking powder
8 oz (16 tablespoons) unsalted butter, softened
2 cups sugar
4 large eggs
1 cup buttermilk
2 teaspoons vanilla extract
1/4 cup poppy seeds

Creme Fraiche Berry Filling
1 pint blueberries
1/4 cup plus 2 tablespoons sugar
2 cups creme fraiche (store bought or homemade)
2/3 cups heavy whipping cream
1 pint raspberries

Creme Fraiche
2 cups heavy whipping cream
2 tablespoons buttermilk

In a plastic container, stir together the cream and buttermilk. Cover with an airtight lid and store at room temperature for 24 hours. Stir and cover. Again, let sit for 24 hours. If the creme fraiche is thick, refrigerate it. If it is still thin, let it stand for another 24 hours.
Preheat the oven to 350 degrees. Grease the bottom and sides of two 9 inch cake pans. Line the bottom with parchment paper.

To make the cake:
In a medium bowl, sift together the flour, baking soda, salt and baking powder.
Beat the butter with the sugar until light, 1 minute on medium-high speed with a stand mixer or 3 minutes with a handheld mixer. On medium speed, add the eggs, one at a time, beating for 30 seconds with a stand mixer or 1 minute with a handheld mixer after each addition. Periodically scrape down the sides of the bowl.

Stir together the buttermilk and vanilla. On low speed, add half of the buttermilk mixture. Mix until incorporated and than scrap down the sides of the bowl. Add half of the dry ingredients. Mix until combined and scrap down the sides of the bowl. Add the remaining buttermilk and dry ingredients in the same manner. Stir in poppy seeds.

Divide the batter between the two pans. Evenly spread it in the pans. Bake the cakes on the middle oven rack until a skewer inserted in the middle comes out clean, about 40 minutes.

Cool the cakes in their pans for 10 minutes. Unmold the cakes by running a small knife around the inside edges of the pans. Place a plate or wire rack on top of each cake and invert the cake and plate. Remove the pans and let the cakes cool completely. Carefully peel off the parchment paper.

Using a serrated knife, cut each cake in half horizontally, making a total of four layers.

To make the filling:
Cook the blueberries and 1/4 cup of sugar in a small saucepan over medium-low heat, stirring often, until the sugar has dissolved and the berries have popped open and become juicy, about 5 minutes. Let cool to room temperature.

Whip together the creme fraiche, cream and remaining 2 tablespoons sugar until stiff enough to hold it's shape but is still smooth.

To Assemble The Cake:
Place a cake layer on a serving platter. Spread one quarter of the creme fraiche filling on the top of the cake. Top with on quarter of the blueberry sauce and then one quarter of the raspberries. Place a second cake layer on the top and spread creme fraiche, blueberry sauce, and raspberries on top as you did with the first cake. Repeat with the third and fourth layers, ending with berries on the top of the cake.

If not serving within an hour, refrigerate the cake. Once removed, let it sit at room temperature for 15 minutes before serving.

Planning ahead:
The cake layers can be made up to 2 days in advance: wrap well in plastic wrap and store at room temperature. The blueberries can be cooked 2 days in advance and kept refrigerated. The creme fraiche and cream should be whipped just before assembling. The cake should be assembled the day it's going to be eaten.