Step 1 Preheat oven to 180°C. Grease and line base and sides of two 18 x 27cm (base measurement) slice pans.

Step 2 Use an electric beater to beat butter and sugar in a bowl until pale and creamy. Add eggs, 1 at a time, beating well after each addition, until just combined. Sift over half the combined flour, cocoa powder and bicarbonate of soda. Add milk. Use a metal spoon to fold until just combined. Sift over remaining combined flour, cocoa powder and bicarbonate of soda. Add water and fold until just combined.

Step 3 Divide mixture among pans. Smooth surface. Bake for 25-30 minutes or until a skewer inserted into centres comes out clean. Cool in pan for 5 minutes. Transfer to wire rack to cool completely.

Step 4 Meanwhile, to make chocolate ganache, place chocolate in a heatproof bowl. Bring cream just to the boil in a saucepan over medium heat. Pour cream over chocolate. Stir with a metal spoon until melted and smooth. Place in fridge, stirring often, for 40 minutes until thickened slightly.

Step 5 To make chocolate filling, use electric beaters to beat butter in a bowl until pale and creamy. Gradually add icing sugar, malted milk powder and milk, in alternating batches, until smooth and combined. With the beater on low, gradually beat in the chocolate until just combined.

Step 6 Place one cake on a wire rack set over a baking tray. Use a palette knife to spread filling over cake. Sprinkle with Tim Tams. Top with remaining cake. Place in fridge for 30 minutes to set.