Toss together pears and 2 tablespoons flour. Set aside.
Melt 3 tablespoons butter in a 9" cast-iron skillet over medium heat. Sprinkle brown sugar evenly in skillet. Fan pear slices in skillet over brown sugar.
Combine 1 cup flour and next 6 ingredients in a large bowl. Cut 5 tablespoons butter into flour mixture with a pastry blender until crumbly. Add buttermilk and molasses, stirring just until moistened. Spread batter evenly over pears, leaving a 1" edge.
Bake at 350° for 45 minutes or until bubbly and browned. Cool in pan on a wire rack 30 minutes before serving. Serve warm with vanilla ice cream.