Preheat oven to 400° F.
In a small bowl, add the sugar and the vanilla seeds that have been scraped from the pod
Rub the sugar and seeds together until the seeds are unclumped and mixed through
Into a bowl sift together flour, baking powder, baking soda, salt, baking spices (if using), and 1 Tbsp of the vanilla sugar.
In a well-seasoned 10-inch oven-safe skillet, melt the butter over moderately low heat before removing from heat.
In a bowl whisk together buttermilk, eggs, and 1 Tbsp melted butter, leaving the remaining butter in the skillet. Whisk into the flour mixture until just combined.
Let batter stand 15 minutes
Peel and core pears and cut lengthwise into ¼-inch-thick wedges
Add pears to remaining vanilla sugar with lemon juice, tossing to coat.
Arrange pears decoratively in butter remaining in skillet.
Sprinkle any remaining sugar mixture over pears and cook over moderate heat until barely tender and sugar begins to caramelize (~ 8 minutes)
Pour batter evenly over pears and bake for 7 minutes
Reduce temperature to 350°F and bake 5-7 minutes more, or until the top is golden and center is firm to the touch
Immediately run a thin knife around edge of skillet
Invert a plate over skillet and invert cake onto plate (keeping plate and skillet firmly pressed together)
Carefully lift the skillet off of the cake and replace any fruit stuck to the bottom.
Serve pancake with warm maple syrup