Butter the inside of a 5-quart round slow cooker, then line it completely with a large sheet of foil and butter the foil. Turn the cooker on low. Sprinkle the cut-up butter and the brown sugar over the foil in the cooker. Add the pears, arranging them in a circle, cut-side down.
Mix the flour, pecans, cornmeal, baking powder, cinnamon, nutmeg and salt in a large bowl. In the large bowl of a mixer, beat the softened butter and granulated sugar on low speed until just blended. Increase the speed to high and beat until light and fluffy, scraping the sides of the bowl as needed, about 5 minutes. Beat in the eggs, one at a time.
With the mixer on low speed, beat the dry ingredients into the butter mixture in two batches, alternating with the milk and beginning and ending with the dry ingredients. Mix on medium speed just until the batter is smooth. Spread over the pears in the slow cooker. Drape paper towels over the top of the slow cooker (not touching the cake) to prevent condensation from dripping onto the cake. Cover and cook on low until the cake sets and the sides brown, about 3 hours. Turn off the cooker and let the cake rest about 20 minutes.
Lift the cake by the foil and place on a rack to cool. Carefully invert onto a platter and peel off the foil. Serve with whipped cream, if desired.