01 - Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper, leaving some overhang for easy removal.
02 - Whisk together flour, baking soda, and salt in a medium bowl. Set aside.
03 - In a large bowl, blend melted butter, light brown sugar, and granulated sugar until smooth. Add eggs and vanilla extract, whisking until fully combined.
04 - Gradually fold the dry ingredients into the wet mixture. Stir until just incorporated; do not overmix.
05 - In a small bowl, cream together softened butter, light brown sugar, and cinnamon until smooth.
06 - Spread two-thirds of the cookie dough evenly into the prepared baking pan. Dollop cinnamon swirl mixture over dough and gently swirl using a knife or offset spatula. Drop spoonfuls of remaining dough on top and gently spread to cover as much surface as possible.
07 - Bake for 23 to 26 minutes, until edges are golden and center is just set. Do not overbake.
08 - Let bars cool completely in the pan on a wire rack.
09 - Whisk powdered sugar, milk, and vanilla extract until smooth. Drizzle glaze over cooled bars, then slice into squares.