Yes you actually can. If you have guests coming over, you can prepare the batter and place in the ramekins up to one day. However if you do this, make sure you cut some parchment paper to size of the ramekin and place on the bottom before adding the batter so the cake doesn't stick. Just cover the ramekins with plastic wrap and refrigerate after that. You can also just keep the lava cake batter in the bowl you mixed in and cover with plastic wrap in the fridge for 2 days before adding to the ramekins. Once ready to bake, remove the lava cakes from the fridge and allow them to come to room temperature before baking.