Prep the pan: Heat the oven to 350°F (175°C). Butter a 9-inch round cake pan with tall sides (or a 9-inch springform wrapped in foil). Line the bottom with parchment.

Make the caramel topping: Melt 4 tablespoons butter in a small pan. Stir in 1/2 cup brown sugar and a pinch of salt until glossy. Pour into the prepared pan and spread evenly.

Arrange the pears: Fan the pear slices over the caramel in a single snug layer. If using, toss pears with a little lemon juice first to keep them bright. Set aside.

Mix dry ingredients: In a bowl, whisk flour, baking soda, baking powder, salt, ginger, cinnamon, and allspice (or nutmeg) until combined.

Cream the butter and sugars: In a separate bowl, beat remaining 4 tablespoons butter with 1/2 cup brown sugar and 1/4 cup granulated sugar until fluffy, about 2 minutes.

Add wet ingredients: Beat in the egg, then the molasses and vanilla. Mix in sour cream until smooth. The batter will look slightly curdled—totally fine.

Finish the batter: Add half the dry ingredients, then the milk, then the remaining dry ingredients. Stir just until combined. Fold in crystallized ginger if using.

Assemble and bake: Spoon the batter over the pears and gently spread to the edges. Bake 35–45 minutes, until the top springs back and a tester comes out clean.

Cool and flip: Let the cake rest 10 minutes on a rack. Run a thin knife around the sides. Invert a serving plate over the pan and flip carefully. Lift off the pan and peel away the parchment.

Serve: Let cool at least 20 minutes before slicing. Serve warm or at room temperature. A dollop of whipped cream or vanilla ice cream is excellent.