Prep the pan: Heat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.
Mix dry ingredients: In a medium bowl, whisk flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
Combine wet ingredients: In a large bowl, whisk melted butter, granulated sugar, brown sugar, and maple syrup until smooth.
Whisk in eggs and vanilla until just combined.
Add buttermilk and dry mix: Stir in half the dry mix, then the buttermilk, then the remaining dry mix. Mix until just combined. Do not overmix.
Fold in blueberries: Toss blueberries with 1 tsp flour to help prevent sinking.
Gently fold into the batter.
Fill and bake: Divide batter evenly among liners (about 3/4 full). Bake 18–22 minutes, until tops spring back and a toothpick comes out clean or with a few moist crumbs.
Cool and brush: Let cupcakes cool in the pan 5 minutes, then transfer to a rack. Stir melted butter and maple syrup together and lightly brush over warm tops.
Sprinkle with cinnamon sugar. Cool completely.
Make the frosting: Beat cream cheese and butter until smooth and fluffy, 2–3 minutes. Add powdered sugar, maple syrup, vanilla, and a pinch of salt.
Beat until creamy and pipeable. Adjust sugar for thickness.
Frost: Swirl frosting onto cooled cupcakes. For a French toast look, finish with a tiny drizzle of maple syrup or a light dusting of cinnamon.