Adjust the oven rack to middle position and preheat the oven to 350F/180C. Butter and flour (or generously spray with a non-stick cooking spray) a 10-cup bundt pan (*see note).
In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda and salt. Set aside.
In a liquid measuring cup (or a small bowl) whisk together the sour cream and milk. Set aside.
In the bowl of a electric mixer fitted with a paddle attachment (or in a large bowl and an electric hand mixer), beat the butter on medium speed until creamy, ribbon-like and slightly lightened up; 2 to 3 minutes.
With the mixer running on medium-low speed, gradually add in the sugar. Raise the speed to medium-high and continue beating together until lightened up in both color and texture; 4 to 5 minutes. The color should turn very pale (almost white) and texture should look fluffy.
Add the eggs and egg yolk, one at a time, beating well after each addition. Add in the vanilla and beat together for one minute.
On lowest speed, add in one-third of the flour mixture and mix until mostly combined. Add in half of the sour cream/milk mixture and lightly stir until almost mixed in. Continue with adding another one-third of the flour mixture, followed by the remaining sour cream and ending with the last third of flour mixture. Mix until just combined and a smooth batter forms. Do not overmix. With a rubber spatula, give the batter one good, final stir to make sure that everything is well incorporated.
Remove 1 1/2 cups of the batter from the bowl and set aside.