Sprinkle yeast and brown sugar over warm water in the bowl of a stand mixer fitted with a dough hook. Let stand for 5 to 10 minutes, or until the yeast softens and begins to form a creamy foam.

Turn the mixer to the lowest setting and slowly add 2 cups flour, 1/2 cup at a time. Add 1/4 cup butter and garlic salt, then slowly add the remaining 2 cups flour and knead until dough is smooth and elastic, 5 to 7 minutes.

Coat a large glass bowl with 1 tablespoon grapeseed oil. Shape dough into a ball and place into the bowl, turning to coat all sides with oil. Spray a piece of plastic wrap with cooking spray place over the bowl, oiled-side down. Cover the bowl with a towel and let rise in a warm area until dough has doubled in volume, about 45 minutes. Punch down the dough and allow to rest for 20 minutes.

After punching down the dough, heat a skillet over medium heat. Add bulk sausage and cook until browned and crumbly, about 5 minutes. Use a slotted spoon to transfer cooked sausage to a bowl, retaining drippings in the skillet.

Add sausage link to the drippings and cook until browned and no longer pink in the center, about 10 minutes. An instant-read thermometer inserted into the center should read 160 degrees F (70 degrees C). Slice sausage.

Preheat the oven to 400 degrees F (200 degrees C). Grease a 12-inch cast iron skillet with the remaining 2 tablespoons grapeseed oil.

Press dough into and up the sides of the prepared skillet. Poke holes in dough with a fork to prevent air bubbles. Spread pizza sauce around the base of the crust. Sprinkle 1/2 of the mozzarella cheese over sauce, then layer with 1/2 of the bulk sausage, 1/2 of the sliced sausage, and 1/2 of the pepperoni. Repeat the meat layers once more, then top with the remaining mozzarella cheese.

Bake on the bottom rack of the preheated oven until the crust is golden brown, about 25 minutes. Brush crust with melted butter, then season with Italian seasoning and garlic powder. Remove pizza from the skillet and let rest for 3 to 5 minutes before slicing.