Chocolate Sheet Cake

For the cake:
Preheat oven to 180ºC (350ºF or Gas 4). Grease a 13×9-inch pan, and set aside.

Whisk together flour, cocoa powder, baking powder, baking soda, salt, and cinnamon. Set aside.

Whisk eggs. Add sugar and whisk until combined. Add vanilla extract, butter, and yoghurt and whisk until combined. Gradually fold in dry ingredients.

Pour te batter into the prepared pan and spread out evenly. Bake for 18–20 minutes, or until a toothpick inserted into the centre comes out clean. Allow to cool completely in the pan on a wire rack.

For the frosting:
Using a handheld or stand mixer, beat butter and icing sugar until light and fluffy. Beat in cocoa powder, milk and vanilla until smooth, and spreadable. Add more milk if needed.

Spread the frosting evenly over the cooled cake (leaving about 1/2 to 1 cup for piping). Transfer the remaining frosting to a piping bag and pipe swirls on each slice of cake. Garnish with sprinkles.