Preheat oven to 350°F. Butter a 12-inch cast iron skillet.
Mix flour and powdered sugar; cut in butter until crumbly. Add water until loose crumble forms.
Press mixture into skillet bottom and sides; bake 20–25 minutes. Cool 25 minutes.
Sprinkle chopped pecans over crust, top with pecan halves.
In saucepan, heat brown sugar, syrup, and butter until dissolved. Stir in cream and vanilla.
Pour hot mixture over pecans.
Bake 20–25 minutes until bubbly and golden. Cool completely before slicing.