preheat oven to 350° and position rack in the middle. spray round cake pan.
cream butter and sugar and spread over bottom and sides of pan. sprinkle pears and walnuts in pan. set aside.
sift flour, baking soda, salt and spices into bowl.
in another bowl, whisk egg, brown sugar, molasses, buttermilk and melted butter. gradually whisk into dry ingredients until combined.
pour batter into pan and bake 35-45 minutes. remove when top of cake is firm and sides pull away from pan.
put plate over pan and flip onto plate. serve warm with whipped cream