Add the sugar and starch to your blender.

Make sure the lid is on tightly, and blend on high speed for three minutes at 20-second intervals. (The blades will get too hot if you blend continuously.) Stop to stir and scrape down the sides as needed. The sugar should be a powder and not at all gritty. Continue blending for 20 seconds at a time until it is a powder.

Store in an airtight container.