STEP 1: Mix psyllium husk powder and water together in a cup, stir well then leave to stand for 5 minutes to thicken into a paste
STEP 2: Combine oat milk, banana, vinegar, oil, vanilla bean paste and the sugars together with the psyllium paste in the bowl of a stand mixer. Mix with the whisk attachment on high speed for around 3 minutes until the ingredients are completely blended to a creamy, thickened consistency
STEP 3: Sift the dry ingredients into the bowl with the wet. Gently combine by cutting and folding together using a spatula or metal spoon until you have a smooth cake batter
STEP 4: Pour cake batter into a prepared sheet cake tin, lined with a greaseproof baking paper sheet. Gently shake the tray from side to side to help the batter settle before placing into a preheated 180c oven. Bake for 35-45 minutes in the centre of the oven until the top is a beautifully golden caramel colour. Test by using a baking skewer from the 35 minute mark, it should come out just clean when removed from the cake. Once cooked, remove form the oven and allow the cake to cool in the tin set on a cooling rack
STEP 5: To make the frosting, cube butter and mix with 100g of the icing sugar in the bowl of a stand mixer (or use an electric hand mixer). Beat until light and fluffy and the butter is fully incorporated. Scoop out a full tub of Crematta™ and add into the bowl of the stand mixer, spoon the remaining 300g of sifted icing sugar and all of the (also sifted) cocoa powder. Fit the bowl’s splash guard (this will stop the icing sugar from creating a cloud as you begin to mix) then starting on a low speed to combine the ingredients, gradually increase the speed of the mixer and add a tablespoon more of icing sugar if required until the icing begins to stiffen slightly and holds its shape ready to be spread (or piped).
STEP 6: Liberally spread the frosting onto the cake, dust with more cocoa if you like. Allow to sit for 15 minutes before then slicing into 9-12 squares and tucking in.