Preheat the oven to 350°F (175°C). Melt ¾ cup unsalted butter gently, without browning it.
Use about 1 teaspoon of the melted butter to lightly grease the inside of a 9-by-13-inch baking dish.
Spoon the entire 21-ounce can of cherry pie filling into the dish. Spread it in a thin, even layer, moving cherries into the corners. Do not drain the sauce.
Sprinkle the dry 15.25-ounce box of yellow cake mix evenly over the filling. Break apart large clumps. Do not prepare the box batter or stir the cake mix into the cherries.
Drizzle the remaining melted butter slowly across the cake mix, reaching the edges and corners. Brush or spread it over dry patches without mixing it into the fruit. If needed, add a little more melted butter directly to dry areas, using no more than 1 cup butter in total.
Bake on the center rack for about 40–45 minutes, until the topping is golden, the edges are lightly browned and the filling bubbles around the sides. If dry patches remain, moisten them with a little melted butter within the 1-cup limit and continue baking until baked through. Cover loosely with foil if the edges brown too quickly.
Let the dessert stand for 10 minutes. Scoop into bowls and serve warm. Vanilla ice cream or whipped cream are optional and are separate from the three ingredients in the dessert.