Make the base layer by putting all of the ingredients into a food processor and pulsing until it is well combined and sticking together.
Press this mixture firmly and evenly into a baking tin with your hands. Use a 30cm x 10cm rectangular or 22cm round tin with a removal base. Place the tin into the freezer while you make the filling.
Make the second layer by placing all of the ingredients into the food processor and blend until very smooth and creamy. Spread this mixture over the base.
Cover and place the tin in the freezer for at least 1 hour to set. You can make this slice 1 or 2 days in advance.
Remove the slice from the freezer 10 minutes before decorating the top with the rose petals and then cutting it into small squares.
Do not let the slice sit at room temperature for too long or it may melt. Store any leftovers in the freezer.
Notes
You could decorate the top of the slice with dried rose petals, crumbled freeze-dried berries, cacao nibs or chopped walnuts.