1. Preheat oven to 160C fan forced.
2. Lightly grease and line a 24cm square cake tin.
3. Combine Flour, sugar, butter and cocoa powder in a bowl.
4. Press over the base of the pan.
5. Bake for 20 minutes, until firm to touch.
6. Remove from oven and cool completely.
7. To make the mousse, place chocolate into a microwave safe bowl with the oil. Microwave on medium power (50%) for 2 minutes, until melted and smooth. Stir well.
8. Whip cream to soft peaks and then add to the melted chocolate mixture. Fold through until combined.
9. Spoon the mousse over the cooled base and use a spatula to smooth the top.
10. Refrigerate for 2 hours or until firm.
11. Cut into slices, garnish with shaved chocolate and serve.