Line the base and two long sides of a 2 lb non-stick loaf tin with a strip of non-stick baking paper or plastic wrap.
To make the granola topping, heat the butter in a saute pan. Stir in the slivered almonds and cook for 2-3 minutes, stirring until almonds just beginning to color. Mix in the pistachio nuts and sugar and cook for 1 minute until the sugar has dissolved. Remove from heat and leave to cool.
For the parfait, pour the coconut milk into a mixing bowl, add the lime rind and juice, honey and sugar and whisk together until smooth. Add the yogurt and whisk briefly until smooth.
Pour about one third of the parfait mixture into the base of the lined tin. Crumble about one third of the blackberries over the top, breaking up with fingertips, then spoon over half the remaining parfait mix and repeat so that all the parfait mixture and all the blackberries are added to the tin.
Level the surface of the mixture and spoon over the cooled granola in an even layer. Freeze for 6 hours or overnight, covering the top with plastic wrap as soon as it is hard.
To serve, take the parfait out of the freezer 15-25 minutes before you are ready to serve it. Allow to defrost slightly then run a wet, round-bladed knife around the sides of the parfait to loosen it. Turn out on to a plate and gently peel off the lining paper or plastic wrap. Decorate with violet flowers and cut into thick slices with a hot knife.