Heat oven to 350°F. Grease a 9×13-inch baking dish with butter.
Whisk milk, half-and-half, beaten eggs, 1 cup brown sugar, granulated sugar, 2 tsp vanilla, cinnamon, salt, and melted butter.
Combine bread cubes with the custard. Fold in 1 cup chopped pecans. Let soak 10–15 minutes.
Spread the soaked mixture in the pan. Scatter remaining 1 cup pecans over the top.
Warm corn syrup, 4 tbsp butter, 1/2 cup brown sugar, and 1 tsp vanilla in a saucepan. Stir until glossy and smooth.
Drizzle the warm pecan pie syrup evenly over the bread mixture.
Bake 40–45 minutes until puffed and the center is just set. Tent with foil if browning fast.
Rest 10 minutes before serving. Spoon warm slices with any pan syrup.