In a large bowl, whisk together the eggs, granulated sugar, and vegetable oil. Add in the Lifeway Vanilla Kefir (because of the vanilla flavor, you can skip vanilla extract), lemon juice, and zest, and continue whisking until smooth. Preheat oven to 350F.
In a separate medium sized bowl, whisk together the flour, baking powder, and salt. Sift the dry ingredients into the wet ingredients and then fold in with a silicone spatula.
Slice 3-4 large strawberries into small cubes, and add the strawberries and blueberries into a small bowl with 2 tbsp of flour. Coat lightly with flour - this helps the fruit to not sink to the bottom of your cake layer. Fold berries into the batter.
Butter two 6 inch cake pans or springform pans, and line with parchment paper on the inside bottoms. Pour batter in two equal parts into each pan. Bake for 45-50 minutes, or until a toothpick inserted into the center of the cakes comes out clean. Let cool. Once completely cool, use a cake leveler to slice each cake into two equal parts, so you have 4 round cake layers.
Add heavy cream to your stand mixer. Use the whisk attachment to whisk the cream to stiff peaks.
Use a large spoon or a spatula to add a layer of whipped cream to the top of your base cake layer. Top with berries and the next cake layer. Add whipped cream and berries again, and add the third layer. Once more add whipped cream and berries, and top with the fourth and final cake layer. Add a few dollops of whipped cream on top and add berries to finish. Slice and enjoy!