Brown the butter in a medium saucepan over medium heat for about 5-7 minutes until deep golden brown and fragrant. Cool to approximately 110°F before using.

Mix the cooled brown butter with granulated sugar and brown sugar until light and fluffy. Add egg, egg yolk, vanilla, bourbon extract, and sour cream, mixing until smooth.

Whisk together cake flour, cornstarch, baking powder, cream of tartar, and salt in a separate bowl; set aside.

Gradually fold dry ingredients into the wet mixture until just combined to avoid tough cookies.

Cover and refrigerate the dough for at least 1 hour to prevent spreading while baking.

Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.

Scoop dough balls about 2 tablespoons in size, placing them 2 inches apart on baking sheets. Bake for 10-12 minutes until edges are golden.

Cool cookies on sheets for 5 minutes before transferring to a wire rack until completely cooled.

Make the glaze by whisking powdered sugar, milk, and bourbon extract until smooth. Adjust milk for consistency.

Dip cooled cookies in glaze, allowing excess to drip off, and let set for 10 minutes before sprinkling with flaky sea salt.