Preheat oven to 325°F and line an 8-inch square pan with parchment paper.
Cream butter and brown sugar together until smooth and fluffy.
Mix in egg, vanilla, powdered sugar, salt, and baking powder.
Add flour gradually, mixing until dough forms.
Press half the dough into the pan to form a base.
Mix brown sugar, cinnamon, flour, and melted butter until crumbly.
Sprinkle evenly over the dough base.
Spread remaining dough evenly on top.
Bake for 30–40 minutes until edges are golden and center set.
Let cool completely before icing.
Whisk icing ingredients until smooth and pour over cooled bars.
Slice into 8 bars and enjoy the buttery, cinnamon sweetness.
