Preheat the oven to 325 degrees Fahrenheit. Oil a 9-inch springform pan, and line the bottom with a 10-inch circle of parchment paper.
To make the topping, combine 3 Tbs butter, ½ cup brown sugar, and cinnamon in a medium saucepan. Melt the butter over medium heat for about 1 minute; then pour the mixture into the prepared springform pan, completely coating the parchment paper. Place the quartered pears on top of the butter-sugar mixture, lining the pieces up tightly in a decorative circle so that none of the bottom shows through.
To make the batter, cut 2 sticks of butter into 1-inch pieces, and put them in a large mixing bowl. Add ¾ cup brown sugar, and cream the mixture on medium speed for 3-5 minutes, until it is smooth and a pale tan color. Add the grated ginger, and beat 1 minute more.
Add the eggs one at a time, beating on low speed and making sure that each egg is fully incorporated before adding another. When all the eggs have been added, slowly pour in the molasses and beat to fully mix.
In a separate medium bowl, sift together the flour, baking powder, baking soda, and salt. Whisk to fully combine.
Alternately add small amounts of flour and buttermilk to the batter, stirring and folding with a rubber spatula until the dry ingredients are just absorbed. Do not overmix the batter.
Carefully transfer the pan to the center rack of the oven, and bake for about 1 hour and 45 minutes, until a skewer inserted in the cake’s center comes out clean. Let the cake cool in the pan for 10 minutes on a wire rack. Cover the pan with an upside-down serving plate; then carefully invert them together. Release the sides of the pan, and lift it away. Gently lift the pan’s base off the cake, and peel away the parchment paper. Allow the cake to cool for a half hour or so, and serve warm, with whipped cream.