Line the bottom of a 6.5-quart slow cooker with heavy duty aluminum foil.
Sprinkle 1/4 cup brown sugar over the foil. Cut 4 tablespoons butter into cubes and sprinkle over the sugar. Arrange the pears in a single layer on top.
In a large bowl, whisk together the remaining sugar, flour, baking powder, cinnamon, ginger, nutmeg, and salt. Set aside.
Melt the remaining 4 tablespoons of butter.
In a separate bowl, whisk together the milk, vanilla, egg, and melted butter. Stir the milk mixture into the dry ingredients until thoroughly combined.
Pour the batter over the pears. Place the lid on the slow cooker and cook on high for 2 1/2-3 hours. When you lightly press the cake in the center with your finger and it springs back, it's done.
When the cake is done, remove the lid from the slow cooker. Carefully remove the cake by gently pulling out the foil overhang and transferring 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.
Serve the cake warm with a scoop of vanilla ice cream or at room temperature.