Place the cashews in a large bowl and cover with boiling water. Let sit for one hour, then drain.
Meanwhile place 450 ml of the soy milk in a small pot. In a small bowl, mix the remaining 50 ml soy milk with cornstarch, until the starch is dissolved.
Bring the soymilk in the pot to a boil and once it boils, stir in cornstarch slurry. Cook for one minute until thickened. The mixture should resemble thick yoghurt.
Remove from heat and add coconut oil. The mixture should still be very warm, so it can melt the oil. Stir until combined.
Place cashews, thickened soy milk, vanilla, and lemon juice in a high speed blender and blend until smooth. Add sugar and blend again.
Transfer to a container and cover with a lid, so the mixture won't start forming a skin.
Let cool for about 30 minutes, then transfer to the fridge and let set over night.
Now the whipped cream substitute is ready to use. Make sure to store the baked goods with vegan whipped cream substitute in the fridge and take out right before serving.