Preheat oven & prepare the pan:
Place a rack in the upper third of the oven and preheat to 350°F (175°C).
Grease a half sheet pan (18x13-inch) with butter or nonstick spray.
Mix the dry ingredients:
In a large bowl, whisk together flour, sugar, cornstarch, baking soda, spices, and salt.
Brown the butter & mix wet ingredients:
In a medium saucepan over medium heat, melt and brown the butter.
Once browned, remove from heat and whisk in the milk powder until smooth.
Add the pumpkin puree and whisk (it may seize, but that’s fine).
Combine wet and dry ingredients:
Pour the buttery pumpkin mixture into the dry ingredients and fold gently.
Add the eggs, vanilla, and sour cream, folding until just combined with no flour pockets.
Bake the cake:
Pour the batter into the prepared pan, spreading evenly.
Bake for 18-22 minutes, rotating halfway, until golden brown and firm but bouncy.
Prepare the frosting:
While the cake bakes, melt and brown the butter in a saucepan.
Remove from heat and whisk in corn syrup, milk, vanilla, and salt.
Gradually whisk in powdered sugar, then stir in pecans.
Frost the cake:
Pour the warm frosting over the warm cake and spread evenly.
Let cool for at least 1 hour before slicing and serving.