Start by browning the unsalted butter in a medium saucepan over medium heat, stirring constantly until golden brown and nutty.
In a mixing bowl, combine the brown sugar and browned butter. Whisk until combined, then add the egg and mix until smooth.
In another bowl, whisk together the flour, baking powder, and cinnamon.
Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.
Fold in the chocolate chips and toasted walnuts until evenly distributed.
Line an 8x8-inch baking pan with parchment paper. Spread the batter evenly in the pan and sprinkle flaky sea salt on top.
Bake in a preheated oven at 350°F (175°C) for 20-25 minutes or until edges are golden and a toothpick comes out with moist crumbs.
Let the blondies cool in the pan for 10 minutes, then lift out using the parchment paper and cool completely on a wire rack before slicing.