Put the soymilk powder, sugar, and water in a saucepan and whisk to combine.
Place the pan over medium-high heat and bring the mixture to a boil. Reduce the heat to low and simmer until thick, stirring often, about 15 to 20 minutes.
Let cool before using in your recipe. It can be stored in an airtight container in the refrigerator for up to 1 week.
Notes
Sugar Note: Plain white sugar or cane sugar yields the most seamless results. If you substitute another type of granulated sweetener, the flavor will be more pronounced and the color will be darker. I haven't tested this with any sugar-free sweeteners - some of those might not work well.
Recipe by Go Dairy Free at https://www.godairyfree.org/recipes/sweetened-condensed-soymilk