In a medium bowl, whisk together flour, baking soda, baking powder, cream of tartar, corn starch, and salt.
In a large bowl, cream the cooled brown butter and brown sugar together until fluffy.
Add egg yolks, vanilla, molasses, and heavy cream to the butter mixture, mixing until pale and fluffy.
Fold in the dry ingredients until just combined. Chill the dough in the fridge for 30 minutes.
Preheat the oven to 350°F and line two baking sheets with parchment paper.
Scoop the chilled dough onto the baking sheets and bake for 9-10 minutes.
After baking, use a cookie cutter or spoon to shape the cookies into perfect circles.
Sprinkle the cookies with flaky sea salt and let them cool before serving.
Get ready to savor every chewy, flavorful bite of these delightful brown sugar cookies!