Baking muffins is fun - filling the paper cups isn't. Especially if the dough is very liquid it is hard not to spill it everywhere.
That's why I designed this Muffin Rocket! It is basically a hopper with three feet to stand on and a slide, which can be inserted into the side of the hopper through a small opening. You only have to insert the slide and pour in the dough and you're all set! Hover the rocket above a paper cup and pull the slide halfway out. The dough (or anything similar) will flow out and can be dosed by pushing the slide back in. I tried this model and after the first round of muffins it had already proven itself useful. Designed to be easily washable.
PLA plastic is allegedly food safe and therefore you should ONLY use PLA plastic with this model. The dough is in contact with the rocket for only a short period of time and at room temperature, therefore it should be relatively safe to use this. But please do so at your own risk! 3D printed objects are generally not recommended for food applications because of their material, the material of the hotend and the small grooves on the objects. Please wash it thoroughly before and after each use. On a brighter side: I didn't die from using this.