Line two baking sheets with parchment paper. Set aside.
To the bowl of your stand mixer, add vegan butter and white sugar and cream them together.
Add the soy milk and vanilla and mix in until well mixed. Set aside.
In a separate bowl, add all purpose flour, baking soda, baking powder, cornstarch and salt and whisk together.
Add dry ingredients to the wet and mix in by hand with a spoon into a thick cookie dough.
Add rainbow sprinkles and mix them in to distribute them as evenly as possible.
Roll the cookie dough into balls and place them evenly over the two parchment lined baking sheets, 10 cookies per tray.
Place the trays into the fridge to chill while the oven preheats.
Preheat the oven to 350°F(180°C).
Place into the oven and bake for 10 minutes.
Remove from the oven and allow to cool and firm up directly on the tray.