Lightly grease the inside of a 6-quart slow cooker with butter or nonstick spray. If using a liner, fit it into the crock before adding any ingredients.
Pour both cans of sliced peaches into the bottom of the slow cooker, including all of the syrup. Spread the peaches into an even layer. Sprinkle the cinnamon and vanilla extract directly over the peaches and stir gently just until the cinnamon is distributed through the syrup.
In a large mixing bowl, combine the dry yellow cake mix with the melted butter. Stir with a fork or spatula until the mixture turns into small, even crumbles, breaking up any large dry clumps.
Sprinkle the crumbled cake mixture evenly over the top of the peaches, covering them completely from edge to edge without stirring the two layers together. Scatter the chopped pecans over the top if using.
Cover the slow cooker with its lid and cook on low for 3 hours, or on high for about 1 hour 30 minutes to 2 hours. The cake is ready once the topping looks set and golden brown rather than pale or wet.
Turn off the slow cooker and let the cake sit uncovered for about 10 minutes before serving. Scoop portions out with a large spoon and serve warm, with any topping of your choice.
Notes
Do not stir the cake mix and peaches together—the layers need to stay separate for the topping to form properly.
Check your slow cooker's actual heat output the first time you make this, as some models run hotter than their settings suggest.
Place a folded paper towel under the lid while cooking to absorb condensation, which helps keep the topping from turning soggy.
Use peaches packed in heavy syrup rather than a "no sugar added" version, since the syrup is what keeps the topping moist as it bakes.
If the edges start to darken faster than the center, rotate the slow cooker's insert a half turn partway through cooking.
Weigh the cake mix box instead of trusting the printed size alone, as box sizes for the same product have changed slightly between production runs.
Store leftovers covered in the refrigerator for up to 5 days. Reheat individual portions in the microwave for 30 to 45 seconds.
Freezing is not recommended, as the peaches release extra liquid as they thaw and the topping turns soft and dense.
For a variation, use a spice cake mix or French vanilla cake mix instead of yellow, or mix in a can of blueberries or fresh raspberries with the peaches.
Avoid leaving the cake at room temperature for more than 2 hours, since the peach syrup makes it prone to spoiling faster than a dry cake would.