Preheat oven to 350 F. Grease a 9 by 13 inch baking pan.To make the cake layer from scratch, cream the butter and sugar on medium-high speed in the bowl of an electric mixer fitted with a paddle attachment until light and fluffy, about 5 minutes.On medium speed, add the eggs, 1 at a time, then the sour cream, vanilla, and lemon zest, scraping down the bowl as needed.
Mix well.Sift together the flour, cornstarch, salt, and baking soda. With the mixer on low speed, slowly add the flour mixture to the butter mixture and stir just until smooth. Finish mixing by hand to be sure the batter is well mixed.
Pour the batter evenly into the prepared baking pan.
Mix the ingredients for the topping and spread evenly on top of the batter.
Bake for about 30 -35 mins and let the cake cool before removing it from the pan.
