Cayo Twist is a small wooden building with a large patio for casual seating. They have a hand-written sign with featured flavor choices, a window to order and another to pick up treats. There's usually a line, but you shouldn't have to wait very long.
The ice cream is soy-based and it's all handmade there. It's good, and seems sent from heaven on most days with the heat and humidity in Belize. You can get waffle cones ~$2 USD with either soft serve or scoops or a quart of ice cream to share or take home ~$5 USD. They also have special treats like sundaes and other creations but I think most people just go for straight ice cream.
It's toward the edge of town in San Ignacio, going towards Guatemala.
Very recommended! Just check the hours carefully since they're pretty limited.
Cons: Limited hours