Turn on the oven to 350 degrees and set a pan (large enough to hold a mixing bowl) half full of hot water on the stove and bring the water to a boil.
Line an 8-inch sponge tin with parchment paper and grease.
Make the mix
In a mixing bowl add the eggs, yolks and sugar and whisk together. Place over the bowl of hot water to allow the mixture to heat up.
Sieve the flour and cocoa and set aside.
Whisk the egg mixture until it doubles in volume and then fold in the butter and flour mixture. Spoon the mix into the cake tin and bake for about 20 minutes until the sponge springs back when pressed.
Source: Thanks https://www.express.co.uk/life-style/food/1271704/queen-elizabeth-ii-birthday-cake-recipe