Note: For the best and most accurate results, I recommend measuring your ingredients using a digital scale. Prepare: Make the vegan mascarpone the day/night before. Prepare the vegan ladyfingers - this can be done on the day (recommended), or a day ahead of making the tiramisu. (See notes 1,2+3)
Make the vegan sabayon: Add the soy milk, rum, sugar, cornstarch, and vanilla extract to a heavy-bottomed saucepan and whisk well until smooth without any visible lumps.
Bring to a simmer over low-medium heat while continuously whisking. Cook for 3-4 minutes until you have a very thick custard. Note: If you're using a gas hob, be extra vigilant and cook it out on the lowest temperature setting to make sure the mixture does not thicken too quickly.
Remove the saucepan from the heat. Add the cold cubed butter to a large mixing bowl and pour the hot custard on top. Whisk vigorously - It will appear split at first but after a few seconds the ingredients will emulsify leaving you with an incredibly smooth and glossy pastry cream consistency.
Cover the sabayon with plastic wrap - this will prevent it from forming a skin. Make sure you leave no space between the custard and the clingfilm. Place it in the fridge to chill completely for 30 minutes to an hour.
Make the mascarpone cream filling: Once chilled, the sabayon will be extremely thick - so much that it will feel wrong! But don’t worry, this is what you need for the pudding to set properly. Add the vegan mascarpone to a blender jug and scoop the sabayon mixture on top of it. If your blender is small, do this in 2 batches.
Blend the filling until smooth and creamy. This should not take very long, under a minute. Set aside.
Make the espresso soak: In a wide shallow bowl or container, mix together room temperature espresso and rum, set aside.
Assemble: Add a thin layer of the mascarpone cream to the base of a 10.5"x7.5" dish (or similar), smooth it out with the back of a spoon or an offset spatula. Individually dip each ladyfinger in the rum-espresso soak for 2-3 seconds on each side and arrange them side by side on top of the cream.
For the most even layers, divide the remaining vegan mascarpone cream into 2 parts. With a spatula, carefully smooth out one part on top of the ladyfingers. Repeat a second layer of soaked ladyfingers on top of this.
Spread the remaining mascarpone mixture on top. Take your time and smooth it out as evenly as possible with an offset spatula.
Generously dust unsweetened cocoa powder on top of the final layer of mascarpone. You can use a pastry brush to carefully brush away any powder from the sides of the baking dish. Refrigerate the tiramisu until set for at least 4-6 hours (for best results - overnight!)
Serving: Carefully mark out your servings with a sharp knife and wipe the knife clean between each cut. I highly recommend using a wide metal spatula to lift out the slices. Then use a large knife or cake scraper to carefully push the tiramisu off of the spatula and onto your serving plate.
Storage: Make sure you store it in the refrigerator until serving. Tightly cover the baking dish with plastic-wrap so that it is not exposed to the air as this can cause it to dry out and absorb odors from the fridge. It will keep in the fridge for 4 days. To freeze, you can add the set tiramisu to an airtight container and freeze for up to a month. Defrost it on a plate in the fridge overnight before serving.