One of my favorite childhood memories is getting a double decker grilled cheese sandwich in the café at our local grocery store. Three level of golden brown edges and two layers of cheese!
Heat a large skillet or griddle over medium to medium-high heat.
Once warm, add in a tablespoons of butter and swirl to coat.
Place a slice of bread down and divide the mozzarella on top before placing a second slice on top.
Once the bottom slice is golden brown, use a spatula to lift the sandwich out of the pan and drop in another tablespoon of butter and then flip the sandwich.
Top the golden brown side with the cheddar jack and place the third slice of bread on top.
Once the bottom is golden brown, lift it out of the pan and drop in another tablespoon of butter and flip the sandwich one final time.
Continue to cook until golden and repeat making the second sandwich.