In a large mixing bowl, combine pumpkin puree, brown sugar, granulated sugar, melted butter, and milk until smooth.
In another bowl, mix flour, baking powder, salt, cinnamon, and nutmeg.
Gradually add the dry ingredients to the wet mixture, stirring until well combined. Add vanilla extract.
Grease the slow cooker and pour the batter into it.
In a separate bowl, mix hot water and caramel sauce, then pour it over the batter without stirring.
Cover and cook on high for 2 to 2.5 hours or until a toothpick comes out clean.
Serve warm, and top with ice cream or whipped cream if desired.