Slow Cooker Banana Upside Down Cake

Difficulty: Intermediate
Servings: 6 to 8
Nutritional Facts Per Serving (1 of 8): Calories 453, Total Fat 16 g, Saturated Fat 9 g, Carbs 75 g, Dietary Fiber 3 g, Sugars 53 g, Protein 4 g, Cholesterol 84 mg, Sodium 134 mg
Total Time: 3 hr 40 min
Preparation: 10 min
Cooking: 3 hr 30 min

Banana Topping

5 tablespoons unsalted butter, cut into small pieces, plus extra to grease the cooker
3/4 cup packed dark brown sugar, plus more for the cooker lining
3 tablespoons dark rum
2 pounds ripe medium bananas (around 6), peeled and halved lengthwise

Cake Batter

3/4 cup cake flour
3/4 teaspoon baking powder
1/2 teaspoon ground cinnamon
1/4 teaspoon ground nutmeg
1/4 teaspoon fine salt
4 tablespoons unsalted butter, softened (1/2 stick)
2/3 cup sugar
1 large egg, at room temperature
1 large egg yolk, at room temperature
2 tablespoons whole milk, at room temperature

Ice cream, optional for serving

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Bananas: Grease the inside of the slow cooker with butter, then fully line it with foil and butter the foil as well. Turn the cooker to HIGH. Sprinkle the butter pieces, brown sugar, and rum evenly over the foil covering the cooker's base. Arrange the banana halves cut side down in a slightly overlapping fashion. Press the bananas gently into the sugar mixture.
Cake: In a large bowl, sift together the flour, baking powder, cinnamon, nutmeg, and salt, then whisk to combine.
In a separate bowl, beat the softened butter and sugar using an electric mixer on low speed until just combined. Increase the speed to high and continue beating until the mixture is fluffy and light, scraping the bowl's sides now and then, about 10 minutes. Incorporate the egg, then the yolk, beating well after each addition.
With the mixer on low, gradually add the flour mixture in three parts, alternating with milk added in two parts, starting and finishing with the flour. Mix briefly at medium speed until you have a smooth batter.
Pour the batter evenly over the bananas and smooth with a spatula. Place a double length of paper towel from one end across the slow cooker's top to line the lid and help seal tightly (see note). Securely cover with the lid and cook on HIGH until the cake edges begin to brown lightly and the center springs back when pressed, about 3 1/2 hours. Turn the slow cooker off and let the cake rest for about 20 minutes.
Lift the cake out of the slow cooker using the foil, and set it on the counter to cool for an additional 30 minutes. Fold back the foil and carefully invert the cake onto a serving plate so the caramelized bananas are visible on top.
Cut into slices or scoop into bowls and serve with ice cream if you like.

*Slow cookers use moist heat during cooking. The paper towel helps catch excess condensation dripping from the lid to prevent it from falling directly onto the cake.