How to Make It
Preheat the oven to 400°. In a bowl, using a fork, stir the flour, cheese, cornmeal, sugar, thyme, baking powder, salt and baking soda. Using a pastry blender or 2 knives, cut in the butter until the mixture resembles coarse meal.
In a small bowl, whisk the buttermilk with the egg yolks. Add the liquid ingredients to the dry ingredients and stir with a fork until a dough forms.
Turn the dough out onto a lightly floured surface and pat it into a 1/2-inch-thick disk. Using a 2 1/2-inch round biscuit cutter, cut out as many biscuits as possible. Transfer the biscuits to a baking sheet. Gather the dough and repeat, handling the dough as little as possible.
Bake the biscuits in the upper and lower thirds of the oven for 18 minutes, or until golden; switch the pans halfway through baking. Transfer the biscuits to racks and let cool slightly and serve.