In a medium bowl, add the applesauce, vanilla sugar, sugar, and salt. Whisk until the sugar is dissolve. Add the cornstarch, mix well.
In a medium saucepan, add non-dairy milk and bring it up to a scald. You should see bubbling around the edges of the pan.
Remove the saucepan from heat and whisk in the applesauce mixture. Once incorporated, return to saucepan to the burner and on medium heat whisk continuously until thickened and a few big bubbles pop, like the hot mud pits at Yellowstone.
Pour the mixture into a bowl, press plastic wrap onto the surface to prevent a skin from forming. Allow to cool to room temperature.
To a large bowl, add room temperature margarine. Use a hand mixer on medium-high speed and beat until fluffy and light in color ~8 minutes.
Add the cooled applesauce mixture to the margarine in 3 batches, mixing well with each addition and scraping the sides of the bowl.
Pipe on to cupcakes using a 1M tip for easy and lovely rosettes.