01 -
Heat your oven to 180°C / 160°C Fan / 350°F. Grease the base and sides of a 33 x 23 cm baking tin (with sides at least 6 cm high) and line it with baking or greaseproof paper.
02 -
Measure out the flour, cocoa powder, and sugar, and add them to a large mixing bowl. Measure the baking powder, leveling off each teaspoon, and mix all dry ingredients together thoroughly.
03 -
Measure the oil, milk, and vanilla extract. Crack the eggs into a small bowl, then whisk them together with the measured liquids until smooth.
04 -
Slowly pour the wet ingredients into the dry ingredients, gently mixing with a spatula or wooden spoon until the batter is smooth and free of lumps.
05 -
Pour the batter into the prepared baking tin and spread it evenly using a spatula. Bake for 45 minutes, checking during the last 10 minutes. Cover with foil if the top darkens too quickly. It's done when a skewer inserted into the center comes out clean.
06 -
Remove the traybake from the oven and let it cool completely in the tin before decorating.
07 -
Melt the butter in a pan, then remove from the heat. Stir in the icing sugar, cocoa powder, and milk until the mixture is smooth and shiny.
08 -
Pour the icing over the cooled cake, spreading it evenly with a spatula. Optionally, add decorations such as sprinkles or chocolate shavings before the icing sets. Once set, cut into squares and serve.