Prep the apples. Zest and juice the lemon into a large skillet, then peel and slice the apples thinly, tossing them in the juice.
Cook the filling. Add both sugars, 5 tbsp flour, spices, and 1/4 tsp salt, cover, and cook over medium heat about 20 minutes, stirring every few minutes, until just tender. Stir in the vanilla.
Fill the dish. Preheat the oven to 375 degrees F and scoop the apples into a 9x13 inch baking dish.
Make the batter. Whisk 2 cups flour, 1 cup sugar, 1/2 tsp salt, and baking powder, then stir in the buttermilk and egg until thick and spreadable.
Top. Spread the batter over the apples, dot with cold butter, and sprinkle with 2 tbsp sugar.
Bake. Bake 45 to 55 minutes until deep golden and bubbling; broil 1 to 3 minutes for a crisper top, watching closely.
Rest. Cool 15 minutes and serve warm.