Start by popping the amaranth. Heat a small pan over medium-high heat for a couple of minutes until hot. Add a tablespoon of the amaranth to the pan and cover. Shake until you hear it start to pop, about 30 seconds or less. Quickly slide the popped amaranth into a bowl. Continue popping the remaining amaranth one tablespoon at a time. The process will go quickly once you get the hang of it.
Stir the yogurt and vanilla together. Add a few spoonfuls of the yogurt into the bottom of each jar, followed by layers of: grapefruit pieces, coconut flakes, honey and the popped amaranth. Then repeat the layering process once more.
These are best served cold and within a day of assembling, otherwise the amaranth will lose its fun crunch.