Preheat oven to 375°F. Lightly grease an 8" square baking pan with butter or cooking spray; if available, line the pan with greased baker's parchment.

Pour the melted butter into the prepared baking dish and tilt to coat the bottom evenly. Sprinkle brown sugar over the butter. Peel, halve and core the pears, and brush them with lemon juice to prevent discoloration. Cut a pear half crosswise into 1/8" thick slices. Keeping the slices together, slide a metal spatula underneath and invert the sliced pear half onto your hand, pressing it to fan out the sections slightly. Place it, rounded side down, over the brown sugar in the baking dish. Repeat with the remaining pear halves. Bake, uncovered, for 15 minutes.

Meanwhile, sift together the flour, baking soda, baking powder, salt, cinnamon, ginger, allspice and nutmeg into a bowl. Stir in the 1/2 cup sugar. In a large bowl, whisk together the eggs, buttermilk, molasses and oil. Add the flour mixture to the egg mixture and stir until blended. When the pears have baked for 15 minutes, pour the batter evenly over the top, and return the pan to the oven for another half-hour. Allow the cake to cool before serving.