Preheat the oven to 350ºF. Place a rack in the center of the oven.
Combine peaches, blueberries, granulated sugar, cornstarch, lemon zest, lemon juice, cinnamon, and salt in an 11″ pie dish or tart pan, stirring to evenly coat the fruit.
Pour 1 box of yellow cake mix on top. Spread into an even layer. Add melted butter directly on top of cake mix, trying to cover as much of the surface with butter as possible. Do not stir or mix. Bake for 45-55 minutes, or until golden brown on top and the cake has puffed up.
Allow the cake to cool slightly for 5 minutes, then serve immediately with vanilla ice cream. Enjoy!
