1. In a medium saucepan, brown the butter over medium heat. You should have 1 cup of butter.
2. Transfer the butter to a large bowl and refrigerate it until solid but still soft enough to stir. Stir the butter every 10 minutes with a rubber spatula to ensure it cools evenly.
3. While the butter cools, whisk together the flour, baking soda, baking powder, cream of tartar, cornstarch, and salt in a medium bowl. Set this flour mixture aside.
4. In a large bowl, cream the cooled brown butter and brown sugar together on high speed for about 2 minutes.
5. Add the egg yolks, vanilla, molasses, and heavy cream to the butter mixture, and mix on medium speed for 1 minute until the mixture is pale and fluffy.
6. Gradually fold the dry ingredients into the wet mixture using a rubber spatula until just combined. Cover the dough with plastic wrap and refrigerate for 30 minutes or overnight.
7. About 10 minutes before the dough finishes chilling, preheat the oven to 350°F (180°C). Line two baking sheets with parchment paper and set them aside.
8. Using a 2tablespoon cookie scoop, portion the dough into balls and place them on the prepared baking sheets, spacing 6 cookies per sheet. You should have enough dough to make about 20 cookies.
9. Bake the cookies for 9-10 minutes, depending on your desired texture (9 minutes for chewier cookies, 10 for crispier ones).
