Line the bottom of a 6.5 quart slow cooker with heavy duty foil.
Sprinkle 1/4 cup brown sugar over the foil. Cut 4 tablespoons butter into cubes and sprinkle over the sugar. Arrange the cranberries and apples in a single layer over the sugar.
In a large bowl, whisk together flour, baking powder, remaining sugar, cinnamon, nutmeg and salt to combine. Set aside.
Melt the remaining 4 tablespoons butter. Whisk together milk, vanilla, egg and melted butter. Stir milk mixture into the flour mixture until thoroughly combined.
Pour the batter over the apples. Place the lid on the slow cooker and cook on high for 2 1/2-3 hours. The cake should spring back when lightly pressed in the center with your finger when done.
When the cake is finished baking, remove the lid from the slow cooker. Carefully remove the cake by gently pulling it out using the foil overhang and transfer the cake to a cooling rack. Allow the cake to cool for 5 minutes, then place a platter on top of the cake and flip over. Peel the foil off the cake slowly.
Cake can be served warm with a scoop of vanilla ice cream, dusted with powder sugar or at room temperature.