Preheat oven to 350°F. Generously spray a 9-inch round (2-inch deep) cake pan with cooking spray. Sprinkle 1 1/2 cups pecans on the bottom of prepared pan.
Melt butter in a small saucepan over medium heat. Transfer 1/2 cup melted butter to a small bowl; set aside. Stir brown sugar and corn syrup into remaining butter in saucepan. Cook over medium heat, whisking constantly, until emulsified and creamy, about 1 minute. Pour over pecans in cake pan; set aside.
Whisk together flour, granulated sugar, pumpkin pie spice, baking powder, salt, and baking soda in a large bowl. Create a well in the center of dry ingredients and whisk in eggs, milk, sour cream, vanilla, and reserved 1/2 cup melted butter until combined. Stir in remaining 1/2 cup pecans. Spoon over pecan mixture in pan.
Bake the cake until a wooden pick inserted in center comes out clean, about 40 minutes. Let cool in pan 5 minutes. Invert onto a serving platter and cool another 30 minutes before serving.
