Line the base and sides of a 28cm spring form cake tin with baking paper.
To make the base place the biscuits in a food processor and process until the biscuits form crumbs. Place in a bowl and mix with the cocoa. Add the melted butter and stir to combine. Pour the mixture into the base of the prepared cake tin and press down to cover the bottom of the tin. Refrigerate while you make the filling.
Place egg whites, sugar, strawberries, lemon juice and vanilla seeds in the bowl of an electric mixer. Beat on high speed for about 6–8 minutes or until the mixture is very thick and fluffy and the sugar has dissolved. To test if the sugar has dissolved stop the mixer and rub a small amount between your fingers. If it is at all gritty the sugar hasn't dissolved so you will need to keep mixing, otherwise it's ready. Spoon the filling over the chilled base, smooth the top, cover with a sheet of baking paper and freeze for at least 4 hours or overnight.
Remove from the freezer and cut into wedges to serve. Serve with fresh strawberries or strawberry coulis.
The cake will keep in an airtight container in the freezer for up to a month.