Step 1

Preheat oven to 150C. Grease and line an 8.5cm deep, 22.5cm square cake pan.

Step 2

Combine butter, chocolate and 400ml water in a large heatproof, microwave-safe bowl. Microwave, uncovered, on HIGH (100%) for 3 to 4 minutes, stirring every minute with a metal spoon, or until smooth.

Step 3

Add caster sugar and vanilla to chocolate mixture. Stir to combine. Add egg. Mix well. Sift flours together over chocolate mixture. Stir until well combined.

Step 4

Pour mixture into prepared pan. Bake for 1 hour 50 minutes to 2 hours or until a skewer inserted into the centre has moist crumbs clinging. Allow to cool completely in pan. Carefully turn cake out. Wrap in plastic wrap, then in foil. Store in an airtight container at room temperature for 5 to 14 days.

Step 5

Using a serrated knife, cut cake in half. Place base on serving platter. Spread one-third of ganache over base. Sandwich together with cake top. Spread remaining ganache over top and sides of cake. Set aside for 24 hours to set. Decorate with fresh roses. To serve, cut cake with a warm knife to prevent ganache from cracking.

Step 6

To make White chocolate ganache: Combine chocolate and cream in a heatproof, microwave-safe bowl. Microwave, uncovered, on MEDIUM (50%) for 2 to 3 minutes, stirring every minute with a metal spoon, or until melted and smooth.

Step 7

Refrigerate for 1 hour, stirring occasionally with a metal spoon, or until thickened to a spreadable consistency.