• Adjust an oven rack to the middle position. Preheat the oven to 350ºF. Butter and flour three 8 by 2-inch round cake pans and line the bottoms with parchment paper.
• In a medium bowl or liquid measuring cup, whisk the eggs, egg yolks, vanilla, sour cream, and buttermilk.
• In a bowl of a stand mixer fitted with a paddle, mix the flour, sugar, baking powder, baking soda, and salt on low until combined. With the mixer running on low, add the butter one piece at a time, beating until the mixture resembles coarse sand. With the mixer still running on low, slowly add half the wet ingredients. Increase the speed to medium and beat until incorporated, about 30 seconds. with the mixer running on low, add the rest of the wet ingredients, mixing until just combined. Increase the speed to medium and beat for 20 seconds (the batter may still look a little bumpy). Scrape down the sides and bottom of the bowl, and use a spatula to mix the batter a few more times.
• Divide the batter into the prepared pans and smooth the tops. Tap the pans gently on the counter 2 times to help get rid of any bubbles. Bake 17 to 29 minutes, rotating the pan halfway through, until the buttermilk cake is golden brown and a wooden skewer or toothpick inserted in the center comes out clean.
• Transfer the pans to a wire rack and let cool for 30 minutes. Turn the cakes out onto the rack, remove the parchment paper, and let cool completely before frosting.

Rhubarb Buttercream:

• Place the rhubarb and water in a medium sauce pan. Bring to a boil, and then let simmer for 15-18 minutes until rhubarb is tender and most the water has evaporated. Place the rhubarb in a food processor, and process until it is a smooth purée. If you’d like to add food coloring to brighten the color of the purée, add a few drops now, and then pulse the rhubarb a bit more, until the food coloring is evenly distributed. Set the purée aside to cool.
• Place the egg whites, sugar, and pinch of salt in a heatproof mixer bowl set over a pot of simmering water. Whisk until sugar dissolves and mixture registers 160 degrees on a candy thermometer. Remove the bowl from the heat, and using a whisk attachment, whisk the mixture on medium speed for 5 minutes. Increase the mixer speed to medium-high, and then whisk until stiff, glossy peaks form, about 6 minutes. Reduce the speed to medium again, and add the butter, 1 piece at a time, whisking well after each addition. When the butter has been completely incorporated, switch to the paddle attachment. With the mixer running on low, add the vanilla, and then the cooled rhubarb puree, and beat until smooth, 3 to 5 minutes. Use immediately, or cover, and refrigerate.

Cardamom Cream:

• Beat the cream cheese and butter on medium-high speed until light and fluffy. Gradually add the sugar and beat until smooth, about 3-4 minutes. Add the vanilla and cardamom, and mix until combined.

To assemble:

• Place one layer of the buttermilk cake on a serving plate. Put half of the cardamom cream on top of the layer, spread evenly over the cake, and then top with another layer of cake. Add the rest of the cardamom cream and spread evenly over the second layer of cake, and then top with the final layer. Frost the cake with the rhubarb buttercream.