P[reheat the oven to 175℃/350℉. Line a 23 x 33cm baking tray with baking spray or a thin layer of butter.
In a large bowl, quickly whisk the sugar, buttermilk and melted butter together until mostly combined.
Add eggs and vanilla essence and whisk until well combined.
Add flour, baking powder, baking soda and salt. Using a spatula, mix until there are no flour pockets left but resist overmixing the batter.
Evenly spread the batter in your baking tray. Note: i9t will seem as if there isn't enough batter but do not worry - it puffs up beautifully. Just make sure it's spread evenly.
Remove the cinnamon filling from the fridge, using two teaspoons, dollop the mixture on top of the batter and roughly combine with the batter. Note: the more you combine the filling with the batter, the less 'holes' there'll be once baked. As the cinnamon filling bubbles during the baking process and forms these craters in the sponge. If you'd prefer less and/or more shallow craters, then combine the 2 more thoroughly.
Place in the oven and bake for 30 minutes or until golden brown on top. Allow to cool. Meanwhile, prepare the icing.