Preheat the oven to 350° F. Combine butter and cream cheese in a bowl with a mixer for 1 minute. Add sugar, egg, and vanilla extract and continue to beat on medium to high speed until light and fluffy, about 5-7 minutes. Add half the flour, all the cornstarch, baking soda, salt, and lemon zest. Add the lemon juice and the remaining flour. Mix to combine until the dough starts to form a ball. Add a little more flour, if needed, for ball formation. Cover and refrigerate the dough at least 2-3 hours or overnight. Prepare a baking sheet with cooking spray. Scoop about 1½ tablespoon cookie dough. Roll into a ball and place it on a baking sheet. Bake for 8-10 minutes. Remove cookies from the oven, and while still hot, slightly press down. Allow cookies to cool completely. Combine powdered sugar and water together in a small bowl. Whisk until ingredients are combined with no lumps. Drizzle on top of cookies and allow to harden.