Preheat oven to 180 degrees celsius (fan-forced)
Grease 2 X 22cm round cake tins.
Mix the bicarb soda, cream of tartar and cornflour on Speed 5 for 10 seconds. Remove the set aside.
Wash and dry the Thermomix bowl.
Place the butterfly into the Thermomix bowl. Add the egg whites and beat on Speed 3 for 2-4 minutes or until stiff (with the measuring cup (MC) removed).
With the butterfly still rotating on Speed 3, add the sugar one teaspoon at a time through the lid. This will take approximately 4-5 minutes.
Reduce to the lowest speed (with the butterfly still inserted) and add egg yolks one at a time through the hole in the lid.
Scrape down the sides of the bowl and remove the butterfly and add the vanilla extract, hot water and cornflour mixture to the bowl and mix on Speed 3 for 5 seconds.
Scrape down the sides of the bowl and mix on Speed 3 for another 3 seconds (or until combined).
Divide the mixture between the two cake tins and place into the oven for 20-25 minutes or until the cakes are golden and a skewer comes out clean.
Leave the cakes in the tins for 5 minutes and then place onto a wire rack to cool completely.