Preheat oven to 350°F (175°C). Grease a 9-inch round cake pan. Line the bottom with parchment if desired.
Make the topping: Melt butter in a small saucepan. Stir in brown sugar, ginger, and salt until glossy. Pour into the pan and spread evenly.
Arrange pears in overlapping circles on top of the caramel.
Mix dry ingredients: Whisk flour, baking powder, baking soda, salt, ginger, cinnamon, and nutmeg.
Cream butter & sugar until light and fluffy (2–3 minutes).
Add eggs, one at a time, then beat in molasses and vanilla.
Alternate dry ingredients and buttermilk, beginning and ending with dry. Mix just until combined.
Spread batter gently over pears.
Bake 40–45 minutes, until a toothpick comes out clean.
Cool 10 minutes, then carefully invert onto a serving plate.