Chef John turned me in the direction of chia pudding, and since my encounter with his recipe, I've tried many different versions. The original, chocolaty is still the best. Even if you don't like chia pudding, this is something of a fun food science experiment in thickening.
0.25 cup chia seeds
1 tbsp brown sugar
1 tbsp white sugar
2 tbsp cocoa powder
0.5 tsp instant coffee
1 cup milk
Mix the dry ingredients. Add the milk and whisk thoroughly. This will take a while, as cocoa doesn't like to mix with cold liquids.
Set aside for 5 minutes, then mix again. Give it another 5 minutes and mix one more time. Cover with foil and refrigerate overnight. Serve cold as a healthy, nutritious alternative to pudding thickened with flour or starch.