If using the mix, mix in a large bowl according to the package instructions. Beat the masa, butter and salt in an electric mixer with the paddle attachment until it forms a ball. If using fresh dough, just add the butter and mix until a ball is formed. Using your hands, shape the dough into 8 discs, roughly 4 inches across and ½ inch thick. Grease a large skillet with cooking spray and set over medium-high heat. Cook the gorditas until lightly browned on each side, about 5 to 10 minutes on each side.