Preheat convection oven to 325˚F.
Prepare two half sheet pans evenly with non-stick spray.
In a small bowl, prepare topping by stirring together pecans, cinnamon, and brown sugar. Set aside.
Blend granulated sugar and softened butter together in a stand mixer using a paddle attachment until smooth and creamy. Add eggs, one at a time, and mix until fully incorporated. Add sour cream and blend until smooth.
Add 2 lbs. of Pioneer® Morning Bounty Buttermilk Pancake and Waffle Mix and blend for 30–60 seconds.
Divide batter evenly into two prepared sheet trays. Sprinkle topping mix evenly on tops of both, then swirl a few spots into batter, leaving most on top.
Bake 15–20 minutes, until set.
While cake is baking, prepare vanilla icing by whisking together the powdered sugar, vanilla extract, and milk until smooth.
When cake has cooled, drizzle the icing on top. Cut and serve.