I always slice my bananas into coins and freeze them in a bag the night before. Frozen banana is the secret to that thick, soft-serve texture without watering everything down with ice. If you forget, even 2 hours in the freezer helps a lot.
Add the frozen banana, kefir or yogurt, milk, honey, and vanilla to your blender. Blend on low first, then increase to high, stopping to scrape down the sides once or twice. You're aiming for a thick, pale, soft-serve consistency, not a thin drink.
I like to tip the blender slightly to see if the mixture holds its shape rather than sliding around. If it's too thick to blend, add a splash more milk, one tablespoon at a time. If it looks thin, add another handful of frozen banana.
Divide the smoothie base evenly between your two bowls using a rubber spatula. I smooth the top flat with the back of a spoon so my piped ghosts have an even surface to sit on and don't slide off to one side.
Spoon the thick Greek yogurt into a small zip-top bag and snip a tiny corner off. Squeeze gently while lifting the bag upward to create a teardrop-shaped ghost, starting wide at the base and pulling up to a soft point at the top.
Press two mini chocolate chips into each yogurt ghost for eyes, and add a small "o" shape mouth if you're feeling extra. Scatter blueberries or blackberries around the ghosts for a graveyard effect, then serve immediately while it's still cold.