For the cake, whisk the eggs and poppy seeds in the bowl of a standing mixer until the eggs are light and the poppy seeds are fully incorporated, about 4 minutes. Cover and refrigerate overnight.
Preheat the oven to 350°F. Thoroughly grease three 8-inch round pans.
Whisk the flour and baking soda in a medium bowl and set aside.
Remove the egg mixture from the refrigerator and whisk to re-combine. Whisk in the sugar, then the kefir. Whisk the flour mixture into the egg mixture.
Divide the batter evenly among the three pans. Bake for 15 minutes, until the edges begin to brown. Remove and cool completely in the pans on a wire rack.
Once cool, remove the cakes from the pans and trim any brown edges. Using a cake tester, poke holes all over the cakes. Brush 1 tablespoon of the Cognac on each layer.
For the frosting, using a stand or hand mixer with the whisk attachment, whisk together the sour cream, sugar, Cognac, and vanilla until very firm.
To assemble, place one cake layer on a serving dish and cover with one quarter of the frosting (about 1 cup) and spread. Place a second cake layer on top and spread with another quarter of the frosting. Place the final cake layer on top. Cover with a generous helping of frosting and use it to cover the top and sides—you may not need all of the frosting. Sprinkle the poppy seeds all over.
Chill for at least two hours, or up to overnight, before serving.