Mash bananas and avocado well using a fork. There should be 2 cups of combined banana and avocado. If you're a little short, add more mashed banana. Add peanut butter and vanilla, beat the mixture or stir with a large spoon, until well combined.
Combine cacao, almond meal, millet, cacao nibs, and walnuts if using. Mix with wet ingredients and beat or stir until well combined.
Pour brownie mixture into a lightly oiled or parchment-lined 8x8-inch baking pan. Bake 30-35 minutes or until the top is no longer wet in the middle. Cool completely.
Cover and refrigerate 2-3 hours before cutting. Store in fridge for up to 5 days.