Grease the inside of a 6-quart slow cooker insert generously with butter or cooking spray. If using a liner, place it inside first, then spray lightly.
In a large mixing bowl, whisk together flour, granulated sugar, baking powder, salt, cinnamon, and nutmeg for about 30 seconds until evenly combined.
Pour milk, melted butter, and vanilla into the dry mixture. Stir gently with a rubber spatula until the batter just comes together and no dry flour remains. Do not overmix.
Peel, core, and chop apples into ½-inch pieces. Pat dry if very juicy, then fold into the batter until evenly distributed.
Scrape the apple batter into the prepared slow cooker and spread into an even layer, pushing gently toward the edges. Tap the side lightly to level.
In a separate bowl, combine brown sugar and boiling water. Whisk until every granule dissolves. For extra richness, stir in a spoonful of caramel sauce.
Slowly pour the brown sugar mixture over the batter without stirring, starting in the center and working outward. Do not mix or swirl.
Cover and cook on low for 2 to 2.5 hours, checking at the 2-hour mark. The top should look set and spring back lightly, with sauce bubbling around the edges.
Turn off the slow cooker, remove the lid, and let rest uncovered for 10 to 15 minutes so the sauce thickens. Scoop into bowls, drizzle with remaining caramel sauce, and serve warm.