Pineapple Upside Down Cake in the Crock Pot

5 tablespoons butter, in small pieces, plus more for the cooker
3/4 cup firmly packed dark brown sugar, plus more for lining the cooker
3 tablespoons dark rum (optional)
8 canned pineapple rings
1/4 cup reserved pineapple juice from canned pineapple rings
8 fresh or frozen cherries, pitted (optional)The pitting isn't optional just the cherries are ;) I use bottled maraschino cherries.
3/4 cup cake flour
2/3 cup sugar
3/4 teaspoon baking powder
1/2 teaspoon ground cinnamon
1/4 teaspoon ground nutmeg
1/4 teaspoon fine salt
4 tablespoons butter, softened
1 large egg, at room temperature
1 large egg yolk, at room temperature
2 tablespoons whole milk
ice cream for serving~ this is optional

Butter the inside of a slow cooker, line completely with foil, and then butter the foil. Turn on high. Sprinkle butter, brown sugar and rum over the foil on the bottom of the slow cooker.

Cover the bottom with the pineapple slices in a slightly overlapping ring pattern, placing one in the center and pressing the pineapple into the sugar. If desired, place one cherry in the center of each pineapple ring.

Sift the flour, sugar, baking powder, cinnamon, nutmeg, and salt into a large bowl; then whisk to combine evenly.

In a separate bowl, beat the egg with an electric mixer. Whisk in the milk and pineapple juice. Add the egg mixture to the flour and blend well to combine completely

Mix briefly at medium speed to make a smooth batter.

Pour the batter over the pineapples and smooth with a spatula to even it out. Lay a doubled length of paper towel from end to end over the top of the slow cooker, to line the lid and create a tighter seal. (The paper towel helps to collect additional condensation from dripping off the lid back onto the cake.)

Cover the cake tightly with the lid and continue to cook on high until the cake begins to brown slightly on the sides and springs back when touched in the middle, about 3 1/2 hours. Turn off the slow cooker and let the cake set, about 20 minutes more.

Using the foil, lift the cake from the slow cooker; set on the counter to cool, about 30 minutes more. Fold back the foil, and carefully invert cake onto a platter so you can see the caramelized pineapples.

Slice or spoon cake into bowls and serve with ice cream, if desired.