Slice the banana and measure out everything before you start blending. Using frozen strawberries helps create that thick, frosty texture that makes the original smoothie so popular. If your banana is very ripe, you may not even need the extra sugar.
Pour the skim milk into your blender, then add the vanilla extract and frozen yogurt. Starting with the liquids helps the blades move more easily and creates a smoother blend.
Place the frozen strawberries and banana slices into the blender. Sprinkle in the sugar if you're using it. I usually taste the smoothie before adding extra sweetness because ripe fruit often does the job on its own.
Top everything with the ice cubes. This gives the smoothie its signature thick consistency. Don't worry if it looks like a lot at first—the fruit will blend down quickly.
Blend on high speed for about 45 to 60 seconds, stopping once or twice to scrape down the sides if needed. The finished smoothie should be creamy with no large chunks of fruit or ice remaining.
Give the smoothie a quick taste. Add a splash more milk if it's too thick or a few extra strawberries if you want a stronger berry flavor. Pour into two chilled glasses and garnish with fresh strawberry slices if desired. Serve immediately for the best texture.
