Bring 1 cup of apple cider to a boil over medium heat, then reduce the heat and allow to simmer for 15 minutes or until the cider has reduced to a half a cup.
Add the chunks of butter into the hot cider and let it melt.
Set aside and allow the buttery cider to cool for 5 minutes.
Add brown sugar to the butter/cider mix and whisk to mix.
Add the egg and whisk until blended.
Add vanilla, cinnamon, apple pie spice, baking powder, salt and mix until well incorporated.
Add oats and flour and stir until well blended.
Pour the batter into the prepared baking dish.
Bake for 25-30 minutes.
Remove from the oven, allow to cool for 10 minutes.
Mix the sugar and cinnamon together in a small bowl.
Brush the top of the blondies with melted butter then sprinkle the cinnamon sugar over the top.
When the blondies have cooled, cut into 16 squares and enjoy.
Store in an airtight container for up to 4 days.
Notes
Sometimes I dice 1/2 apple and add it to this recipe for added fiber, or add 1/2 cup of dried cranberries or raisins.