Preheat the oven to 350℉. Melt the butter in a small pot over low heat.
Slice the French loaf into 1-inch pieces then place into a large mixing bowl. Pour about 3/4 of the butter over the bread, toss to coat.
Place the raisins and rum into a small bowl then stir.
Add the milk, sugar, vanilla, pumpkin spice, and lemon juice to a blender. Process on high until smooth. Pour the mixture over the bread, stir, then allow to soak for 15 minutes.
Separate the raisins from the rum. Reserve ¼ cup of raisins and the remaining rum for the sauce. Fold the other ½ cup of raisins into the bread pudding mixture.
Drizzle the remaining butter into the bottom of a 9x9-inch casserole dish. Use a kitchen brush to coat the base and sides of the dish with butter. Transfer the soaked bread to the prepared casserole dish.
Place the casserole into the oven and bake for 45-60 minutes, or until the edges are crisp and the top is golden brown. Remove from the oven and cool for 10 minutes.
In a small mixing bowl, stir together the powdered sugar, remaining raisins, and rum. Continue to stir until smooth. To serve, scoop the bread pudding onto a serving plate and drizzle the rum sauce over top.