Step 1

Preheat oven to 160°C. Grease and line a 23cm round cake pan with baking paper.

Step 2

Combine cocoa with 200ml boiling water and set aside to cool. Combine flour, sugar baking powder and a pinch of salt in a bowl.

Step 3

Whisk oil and egg yolks in a bowl. Stir in the cooled cocoa mixture, then whisk into the dry ingredients until smooth.

Step 4

Whisk eggwhites and cream of tartar to stiff peaks, then fold into cocoa batter. Pour mixture into prepared pan. Bake for 55 minutes or until a skewer inserted into the centre comes out clean. Cool in pan for 20 minutes, then invert onto a wire rack and cool completely.

Step 5

For praline, cook sugar, without stirring, in a pan over medium heat for 3 minutes or until a dark caramel forms. Quickly stir in hazelnuts, then spread over a baking paper-lined baking tray and sprinkle with a pinch of salt. Cool. Break into shards. Transfer to a food processor and pulse to coarse crumbs. Transfer half to a bowl and set aside. Add oil to remaining crumbs in food processor and whiz to a paste.

Step 6

For the cornflake layer, place chocolate in a heatproof bowl set over a pan of simmering water (don't let bowl touch water) stirring until melted. Stir in praline paste. Fold in cornflakes and set aside.

Step 7

For the chocolate buttercream, place eggwhites and sugar in a large heatproof bowl. Set over a pan of simmering water and whisk for 3 minutes or until sugar dissolves and mixture looks opaque.

Step 8

Remove bowl from heat, add vanilla and whisk in an electric mixer on high speed until cooled. Gradually beat in butter until thick and pale. If it looks curdled, continue whisking until it comes back together before adding remaining butter. Place chocolate in a heatproof bowl set over a pan of simmering water (don't let bowl touch water) and stir until melted. Cool for 6 minutes, then fold into buttercream.

Step 9

Slice sponge horizontally into three. Spread base with 1/2 cup buttercream and spread over half the cornflake mixture. Repeat with second sponge, 1/2 cup buttercream and remaining cornflake mixture. Top with final sponge. Spread remaining buttercream over whole cake, then chill for 30 minutes.

Step 10

For the icing, place all the ingredients in a pan with 50g reserved praline and gently melt over low heat. Cool to room temperature but don’t let it set. Ice the top of the cake, allowing icing to drip down. Top with Ferrero Rocher, some wrapped in gold leaf, if using. Scatter remaining praline around base of cake. Set cake aside for 20 minutes before slicing and serving.