Preheat your oven to 350°F (175°C) and line a 9x13 inch baking pan with parchment paper.

Cream together softened butter, granulated sugar, and brown sugar until fluffy. Beat in the eggs one at a time.

Whisk flour, baking powder, and salt in a separate bowl, then mix the dry ingredients into the wet ingredients until combined. Reserve 1 cup of dough.

Press remaining dough into the bottom of the lined baking pan and bake for 10-12 minutes until edges are golden.

Prepare the filling by combining melted butter, brown sugar, cinnamon, and cardamom until thick. Pour over the baked crust.

Dollop the reserved dough over the filling and swirl with a knife. Bake for another 28-32 minutes until set.

Let cool for 15-20 minutes. Prepare frosting by blending all frosting ingredients until smooth and spreadable.

Frost the bars once cooled, then slice and serve as desired.