I minimize nuts because of high fat and protein. The following are Kcal dense from complex carbs. And they are low protein, have very low fat, and have other beneficial nutrients.
Ragi - the most dense at ~3.8cal/g, 9:1 carb to protein, very low fat - note, it must be finger millet, other millets can't compete. interesting fact for you since you have some S. Indian ancestry, ragi is big in Karnataka and I think other parts of south India, too. Many recipes including an easy to make porridge, sprouted is best. If you added ~90g of ragi, a little less than 1 serving, you'd get ~500 calories.
Chestnuts - ~ 1.8cal/g, almost 10:1 carb to protein and almost no fat, even less than ragi. How is this a nut with so little protein and fat? Fun fact: deer also love chestnuts.
Purple sweet potatoes - ~ 1.6 cal/g, a whopping 15:1 carb to protein and zero fat