Grease glass 8x8 baking pan with butter.
Butter slices of bread. Discard heel ends and trim crusts off bread. If you like, reserve trimmed crusts for making bread crumbs or egg casseroles. Cut buttered bread into triangles and arrange in a single overlapping layer into prepared dish.
When first layer of bread is complete, sprinkle with 1 tsp to 1-1/2 tsp sugar. Sprinkle with a small handful of raisins (this can be varied to your preference).
Repeat layers 2 more times, for a total of 3 layers. You will have leftover bread, but should use most of the butter.
In a separate bowl, whisk together eggs and 1-1/2 Tbsp sugar. Whisk in milk and vanilla.
Preheat over to 350 F with rack in the middle to lower middle of the oven.
Carefully pour custard mixture over the bread layers. Let stand for 15-20 minutes.
Stand glass baking pan with bread pudding in a roasting pan of water. The water should reach halfway up the sides of the baking pan. Poach in oven about 90 minutes until the custard is set and is a light, golden brown. A knife inserted in the middle should come out clean. Serve warm, with rum sauce.
Store leftovers in the fridge, 2-5 days in a covered container.