Preheat the oven to 170℃ (150℃ fan)/325°F/gas mark 3.
Mix the butter and biscuits together , then press down well into the base of a greased 20cm (8in) loose bottomed or spring clip cake tin.
Chill in the freezer for 10 minutes or the refrigerator for 30 minutes.
Place the ricotta, cream cheese, crème fraîche, vanilla paste, sugar eggs and flour in a food processor and whizz until well blended or beat together with an electric whisk or wooden spoon.
Pour over the biscuit base. Bake for 1 hour or until firm around edge but still slightly soft in the middle.
Turn off the oven and leave the cake in the oven to cool with the door left slightly open.
Remove from the oven and carefully take the cake out of the tin and transfer to a serving plate. Chill until required.
Meanwhile make the coulis if desired. Purée 150g (5oz) strawberries together with the icing sugar in a jug blender, food processor or with a hand blender. Chill until required.
When you are ready to serve, pipe the cream around the edge of the cheesecake Toss the remaining strawberries in a little strawberry coulis and arrange or pile on top of the cheesecake.
Serve the cheese cake with the strawberry coulis on the side.