Start by making a ½ cup of strong black coffee. You can use instant coffee, espresso capsules or ground coffee. Leave to brew and cool. Preheat the oven to 180C/ 350F. Line a shallow baking tray with baking paper. To make the sponge, separate 3 eggs. Put 3 yolks into a large bowl and the whites into a small bowl. Pour the almond milk and avocado oil in with the egg yolks. Whisk to a smooth consistency, then add in the almond flour, psyllium husk, bicarbonate soda and salt. Mix to combine.
Add the egg whites to the bowl of a stand mixer with the whisk attachment. Whisk on medium speed for 1 minute until the eggs start to froth. Add in the lemon juice, vanilla extract and 1/4 cup graulated erythritol and keep whisking until stiff peaks form, about 5-8 minutes.
Fold in the egg whites into the egg yolk mixture in small batches until a smooth consistency is achieved. Folding is important to keep the air in the mixture.
Pour into the prepared baking tray. Use a spatula to evenly spread the mixture. Bake in the middle of the oven for 15 minutes or until evenly golden brown. Leave to cool.
Beat the yolks and 1/3 cup powdered sweetener in a bowl of a stand mixer with the whisk attachment, or use an electric beater on medium-high speed for 8 minutes or until it changes from yellow to a very pale yellow. Add the vanilla and mascarpone, beat for a minute until just combined and silky smooth. Transfer mixture to a bowl and set aside. If you prefer a sweeter dessert, you can increase the powderd erythritol to 1/2 cup.
Clean the stand mixer bowl and whisk to use again. Pour the egg whites into the mixer bowl and beat until stiff peaks form. About 5 minutes on a medium-high speed, they will be quite foamy.
Use a spatula to gently fold the cream mixture into the egg whites in small batches. Take care to not beat all the air out. It may look a little lumpy but these will settle once assembled.
Pour the coffee into a small bowl and stir through the spiced rum if using. You can make it as strong as you like! Use the top rim of the dish as a guide to cut out 2 circular disks in the sponge. Reserve the offcuts to form another layer.
Assemble the tiramisu by first dusting a heavy layer of cocoa powder at the base of the serving dish. Place the first sponge disk at the base and use a pastry brush or spoon to thinly soak the coffee mixture all over. Spread a thick layer of cream over the sponge and dust again with cocoa powder. Use the offcuts to piece together the middle layer. Repeat this sequence again with the coffee, cream and cocoa powder until the final layer of cream and cocoa powder have been applied. Place in the fridge for at least 3-4 hours or overnight is best before serving.