Preheat oven to 350° F. Grease an 8-inch round cake pan and line the bottom with parchment paper.
In a small bowl, whisk together the egg yolks, sour cream, milk, and vanilla.
In a separate bowl, sift together the flour, cornstarch, baking powder, baking soda, and salt.
In a third bowl, beat the butter and cream cheese together until smooth and well combined. Add the lilac sugar and lemon zest; cream until fluffy. Add about one third of the flour mixture to the butter mixture and blend until almost incorporated. Then add about one third of the wet ingredients and fold in. Repeat alternating the dry and wet ingredients until all ingredients are just blended.
Pour the cake batter into the prepared pan and bake until lightly golden and center springs back when touched, about 35-40 minutes.
Allow to cool completely. Invert and remove from pan, removing parchment paper from bottom. Place on a cake plate.