Stir butter, brown sugar, and ½ cup syrup in heavy medium saucepan over medium heat until melted and smooth. Transfer to 13x9x2-inch glass baking dish. Sprinkle pecans over butter mixture.
Whisk eggs, half and half, cinnamon and vanilla in large bowl to blend. Dip bread slices into egg mixture and arrange in single layer in prepared dish. Pour remaining egg mixture over bread slices in dish. Cover with plastic wrap and refrigerate overnight.
Preheat oven to 350?F. Bake French toast uncovered until golden brown and puffed and knife inserted into center comes out clean, about 40 minutes. Cut into 6 pieces. Serve pecan-side up with additional warm maple syrup.