In the bowl of an electric mixer fitted with the paddle attachment, mix together brown sugar, butter, and eggs until very light and fluffy. Add sweet potato and vanilla; beat to combine. Add flour, milk, bourbon, baking powder, pie spice, and salt; mix until just incorporated. Lightly spray a 10 or 12 cup Bundt pan with cooking spray. Sprinkle bottom and up sides of the pan with pecans. Pour in cake batter.
Bake until a toothpick inserted in the center of the cake comes out clean, 50 to 60 minutes. Let cool 10 minutes. Remove from pan; cool completely on wire rack. Brush cake with bourbon syrup. Serve cake slices with any remaining syrup.