Step 1

Using an electric mixer (or stand mixer fitted with the paddle attachment) on medium speed, beat granulated sugar, butter, and salt in a large bowl, scraping down sides, until well combined (butter does not need to be fluffy), about 3 minutes. Beat in egg. Reduce mixer speed to low and gradually beat in flour. Add corn syrup, milk, and vanilla and beat on low speed, scraping down sides, until combined and no streaks of syrup remain.

Step 2

Preheat oven to 350°. Butter a 13x9" pan. Whisk flour, baking powder, and salt in a medium bowl. Using an electric mixer on medium speed, beat granulated sugar, brown sugar, and remaining ¾ cup butter in a large bowl, scraping down sides, until well combined (butter does not need to be fluffy), about 3 minutes. Beat in eggs one at a time until incorporated (mixture will look loose and broken, that’s okay), then beat in vanilla. Reduce mixer speed to low and gradually beat in dry ingredients in 3 additions, alternating with sour cream in 2 additions, beginning and ending with dry ingredients. Scrape batter into prepared pan, smoothing top with a spatula (if you have an offset spatula, use it!). Dollop goo over batter. Using spatula, gently and evenly spread goo over entire surface.

Step 3

Bake cake until browned, edges begin to pull away from pan, and middle is slightly wobbly, 45–55 minutes (cake will appear to boil at about 30 minutes, but don’t be frightened; keep baking!). When fully baked, it will look like the surface of the moon, with craters and valleys of goo and mountains of cake. Let cool 1 hour in pan, then dust with powdered sugar.