Combine 1 cup of water and 1/8 cup of cornstarch in a blender.
Blend on high speed until the mixture is smooth, which takes about a minute.
Transfer the blended mixture to a 2 1/2-quart saucepan.
Heat over medium-high heat, stirring constantly to prevent sticking.
Once the mixture thickens, add 3/4 cup of sugar and mix well.
Remove the saucepan from the heat.
In a separate bowl, mix 1 cup of mayonnaise, 1/4 cup vinegar, and 1/8 tsp garlic powder.
Combine with the cornstarch mixture until smooth.
Let the dressing cool completely before transferring it to a container for refrigerating.