Preheat the oven to 350°F (180°C). Have ready an 8x11-inch oven dish (ceramic or glass).
Open 2 cans cherry pie filling and dump them in the dish, spreading to cover the bottom. For homemade filling, see below.
Scatter 3 cups chocolate cake mix with a spoon, making sure it covers all the cherries. Smooth slightly but don't mix and don't press too much. We want to flatten it a little so the butter slices are easier to arrange.
Thinly slice ⅔ cup unsalted butter and arrange them on the cake mix. You should cover most of it so that when it melts in the oven, all (or most) of the cake mix is moistened. Alternatively, melt the butter and pour it over the chocolate cake mix in a large bowl. Mix with a spoon until the mixture is crumbly. Sprinkle it over the cherry filling.
Bake on the middle rack for 45 minutes. Check by lifting the chocolate top here and there. If there are very dry spots, mix it a bit with the juices in the bottom. Otherwise, they might never moisten.
Bake for 15 minutes more (1 hour in total) until the top is dry and the filling bubbles around the edges.
Serve warm, plain or with whipped cream or a scoop of vanilla ice cream.
Store leftovers in the fridge, covered with plastic wrap. You can also freeze them, well covered or in an airtight container. Defrost at room temperature.