Preheat the oven to 425ºF (218ºC). Grease and flour four 6oz ramekins or six wells of a cupcake pan (no need to use liners). Set aside.
In a small bowl, stir together the cake mix, water, egg, and vegetable oil with a spatula until smooth. Pour the batter evenly into the prepared ramekins (about ¼ cup of batter in each) or cupcake wells (about 2 and ½ Tablespoons in each).
Place the ramekins on a baking sheet and bake lava cakes for 9-10 minutes or 5-6 mins for a cupcake pan. Allow cakes to cool for 1 minute, then invert a plate on top of each ramekin and flip over (use an oven mitt-- the ramekins will be very hot). If using a cupcake pan, either scoop cakes out with a spoon, or invert a plate or serving platter large enough to accommodate all of your cakes.
Add desired toppings* and serve lava cakes immediately. Leftovers can be kept in the refrigerator, covered, up to 3 days. Warm in the microwave for 10-15 seconds before serving. Lava cakes freeze well, up to 3 months. Allow to cool completely before freezing, Thaw in refrigerator overnight and warm in microwave before serving.