Add 1.5 cups frozen mixed berries and 1 frozen banana to a high-speed blender.
Pour in 1/3 cup milk to start. Add 1/4 cup Greek yogurt if using.
Add 1 tablespoon nut or seed butter and 1/2 teaspoon vanilla extract, if desired.
Blend on low, then increase to medium, using a tamper to push fruit toward the blades.
If it struggles, add milk 1 tablespoon at a time. Aim for a thick, soft-serve texture.
Taste and adjust. If you prefer sweeter, add 1 to 2 teaspoons honey or maple syrup and blend briefly.
Scrape into a wide bowl.
Smooth the top with a spoon.
Add toppings: a handful of fresh berries, banana slices, a sprinkle of granola, and a pinch of chia or coconut flakes. Serve right away.