Draping paper towels over the top of the slow cooker helps absorb extra moisture and keeps this sweet, caramelized cake from getting soggy. We line the insert with a double layer of foil to prevent burning around the edges.
Butter the inside of a 4-quart slow cooker insert. Line it completely with a double layer of foil, then butter the foil. Cut up 4 tablespoons of the butter and sprinkle in the bottom of the insert, along with the brown sugar and rum. Arrange the pineapple slices in overlapping circles to completely cover the bottom of the insert; press into the sugar mixture.
Combine the flour, pecans, baking powder, cinnamon, nutmeg and 1/2 teaspoon salt in a medium bowl.
Combine the remaining 6 tablespoons butter and the granulated sugar in the bowl of a stand mixer fitted with a paddle attachment (or in a large bowl if using a hand mixer) and beat on medium-high speed until light and fluffy, about 3 minutes. Scrape down the sides of the bowl and beat in the eggs and vanilla until incorporated.
Reduce the speed to low and beat in the flour mixture until just incorporated. Add the milk, increase the speed to medium-high and beat until smooth, about 30 seconds. Evenly spread the batter over the pineapples in the slow cooker.
Drape a double layer of paper towels over the top of the slow cooker (without touching the cake) to prevent condensation from dripping from the cover onto the cake. Cover and cook on low until a toothpick inserted in the center comes out clean, 3 to 4 hours. Turn off the slow cooker and let the cake rest for 20 minutes.
Lift the cake by the foil and place on a rack to cool (it can be served warm or at room temperature). Carefully invert onto a platter and peel off the foil.
Whip the cream until soft peaks form. Fold in the confectioners’ sugar and chopped cherries. Serve slices of cake with a dollop of whipped cream and a cherry on top.