Coat a 1-1/2-quart soufflé dish with 1 tbsp of the melted butter. Add bread and raisins to dish. Pour milk over top; stir to combine. Set aside.
Combine ricotta, sugar, eggs, sour cream, lemon zest and vanilla. Pour ricotta mixture over bread; mix lightly. Drizzle remaining tbsp butter over bread and sprinkle with cinnamon. Cover dish with aluminum foil; place in slow cooker. Fill slow cooker with boiling water to reach 1 inch up the side of soufflé dish.
Cover top of slow cooker with a dish towel; top with lid. Slow cook on HIGH for 2-1/2 hours or until internal temperature of pudding registers 140°. Cool at least 20 minutes.