Preheat oven to 175°C (350°F). Line an 8-inch tin with parchment.
Melt butter, then stir in brown sugar, egg, and vanilla.
Add flour, baking powder, salt, and cinnamon. Mix until just combined.
Fold in apple pieces.
Spread batter in the tin and bake 25–30 minutes until golden.
Allow to cool on a wire rack.
Make the glaze - mix the icing sugar, cinnamon and milk together in a large bowl. Drizzle over the blondies and enjoy!