Set a wide skillet over medium heat. Add 2 tablespoons butter, allow it to melt and then cook to a golden brown hue, about 5 minutes. Add pears and sauté until pears are tender and butter turns into a thick, deep golden sauce, 5 minutes more. Fold in 1/4 cup sugar brown sugar; cook until sugar is dissolved, 3 - 4 minutes. Remove from heat and set aside.
Ina shallow dish, combine remaining 1/4 cup brown sugar, cinnamon, and allspice.Spread remaining 4 tablespoons butter on both sides of sliced bread. Press each side into cinnamon and sugar mixture, enough to lightly coat each slice.Arrange in 8 x 8-inch baking pan.
Ina small bowl, whisk to combine eggs and milk. Fold cooked pears, and all accumulated juices and syrup, into the mixture, and carefully pour over bread.Set in fridge overnight (or a minimum of 2 hours).
Preheat oven to 350 degrees F. Bake 40 - 50 minutes or bread pudding is set and starts to pull away from the sides. Cool 30 minutes.
Dust with powdered sugar and serve with a dollop of lightly sweetened whipped cream.