• Preheat the oven to 160°C fan (320°F).
• Mix sugar and eggs in a large bowl.
• Add flour, cocoa, vanilla, baking powder, and cooled melted butter.
• Top the batter with the boiling water (a little at a time) as you whisk.
• Keep whisking for 1-2 minutes (I use a handmixer) until there are no lumps.
• Pour the batter into a lined or greased sheet pan.
• Tap the cake on the counter to remove trapped air bubbles.
• Bake for 20 minutes until the center is just baked.
• Allow to cool completely in the pan.

Frosting

• Mix the butter and half of the powdered sugar with a hand mixer.
• Add the remaining powdered sugar, cocoa, and milk, and whisk until fluffy and light (about 2 minutes).
• Spoon the icing onto the completely cooled cake.
• Serve.