Preheat oven to 350F. Line an 9-by-10-inch baking pan with parchment paper so the edges of parchment paper stick up (this way you can lift the bars out to cut when they are done); set aside.
Reduce the apple cider: In a sauce pan, heat the apple cider over medium heat, until it comes to a low boil. Reduce the flame to a simmer until the apple cider has reduced to 1 cup (about 15 minutes)
In a large microwave-safe bowl, melt the butter.
Whisk in the brown sugar and vanilla. Then whisk in the eggs.
Add the flour, salt, baking powder, cinnamon and nutmeg and whisk until just combined, don’t overmix.
Fold in the caramel cups (optional).
Spread the batter out into prepared pan, smoothing the top lightly with a spatula.
Bake for about 20 to 25 minutes, or until bars have firmed up in the center and edges are set. About 5 minutes before the bars are done, sprinkle with cinnamon sugar topping.
Allow bars to cool in pan for about 15 minutes before lifting them out of the pan. Then allow them to fully cool before cutting.