Preheat an oven to 350°F (175C/155C Fan) and prepare your baking tin. Grease the base and sides and line with parchment paper. Set aside.
Place the egg whites in a large mixing bowl with the cream of tarter. Whisk this together using a handheld electric beater until you reach soft peaks.
Slowly add the caster sugar a tablespoon at a time, continuing to whisk on high until glossy stiff peaks form. Set aside while you make the cake batter base.
In a separate large mixing bowl, stir together the flour, corn starch, brown sugar, baking powder, cinnamon and nutmeg.
In a jug, whisk together the egg yolks, water and vanilla extract. Add this to the bowl of dry ingredients and whisk to combine.
Add a spoonful of the meringue to the cake batter base and stir to loosen the mixture. Then add the remaining meringue and fold through gently to combine.
Pour the mixture into the prepared baking pan and spread carefully into the corners, smoothing the top evenly.
Bake in the centre of the preheated oven for about 20 minutes, until well risen and beginning to lightly brown. The sponge should not be sticky and should spring back lightly to the touch.
Remove from the oven and allow to cool in the tin for five minutes.
Dust the top of the cake with powdered icing sugar and invert onto a piece of parchment paper on a wire cooling rack (see the blog post for step by step tips).
Peel the parchment paper lining carefully off of the cake and dust lightly with powdered sugar. This side will be the outside of your finished roll cake. Allow to cool for five minutes.
Flip the cake onto a clean sheet of parchment paper, adding another light dusting of powdered sugar if needed. Use the parchment paper to roll the cake from the short edge nearest you ensuring that parchment paper is between the cake in the rolling to avoid the cake sticking to itself. Set aside while you make your cream filling.