Lightly grease the inside of a heatproof baking dish with butter. Make sure the dish fits comfortably inside your slow cooker with enough room around the sides for the lid to close. Add a trivet or folded kitchen towel to the bottom if needed to prevent the dish from sitting directly on the heating surface.
In a large mixing bowl, whisk together the pumpkin purée, eggs, maple syrup, milk, and heavy cream until smooth. Whisk gently until the eggs are fully incorporated and the mixture has a consistent texture.
Add the brown sugar, if using, along with the vanilla extract, cinnamon, ginger, nutmeg, cloves, and salt. Stir until the spices are evenly distributed. Taste the mixture before adding the eggs if you want to adjust the sweetness or spice level; do not taste the raw egg mixture.
Pour the pumpkin mixture into the greased heatproof dish. Smooth the surface with a spatula. Cover the dish loosely with foil to prevent condensation from dripping directly onto the pudding.
Place the filled baking dish inside the slow cooker. Carefully pour hot water into the slow cooker insert until it reaches approximately halfway up the sides of the baking dish. Avoid getting water inside the pudding.
Cover the slow cooker with its lid and cook on LOW for approximately 2–3 hours. Cooking times vary by slow cooker and dish size, so begin checking around the 2-hour mark. The pudding should be mostly set around the edges, with only a slight wobble in the center.
Carefully remove the hot dish from the slow cooker using oven mitts. Remove the foil and allow the pudding to cool for 20–30 minutes. It will continue to firm up slightly as it cools.
Spoon the pudding into individual serving bowls or serve directly from the baking dish. Add whipped cream, a light sprinkle of cinnamon, chopped toasted pecans, or a drizzle of maple syrup. Enjoy it warm, or refrigerate it for a chilled custard-style dessert.