Combine the chia seeds and cacao powder in a small resealable container. Pour in ½ cup of soymilk and stir to incorporate. If you're using unsweetened soymilk, stir in sweetener of choice, to taste.
Let the mixture sit at room temperature for about 10 minutes, then give it another good stir, put on the lid, and refrigerate for at least 2 hours. The longer you refrigerate the pudding, the thicker it will be.
When you're ready to assemble your parfait, whisk together the peanut butter with 2 tablespoons of soymilk in a small bowl. If the mixture is too thick, whisk in an additional tablespoon of soymilk.
Layer the chia pudding with the peanut butter sauce and banana slices in a serving dish or jar. Top with cacao nibs, if using, and serve immediately.