Line the base of a 10-11" spring form cake tin with baking paper. This will help you get your cake out later.
Put your graham crackers into a food processor / blender and blend until it's small crumbs.
Add the melted butter, and blend for a couple of seconds until it starts to come together.
Press firmly into the base of the prepared tin. You don't want it to be a thick layer, you just want it to cover the bottom of the tin. Pop in the fridge while you make the topping.
Place one egg white, 1/2 cup sugar, sliced strawberries, 1 1/2 tsp lemon juice and 1/2 teaspoon vanilla extract in the clean, dry bowl of an electric mixer. Beat on high speed for about 6-8 minutes, until the mixture is very thick and fluffy. The beating should increase the volume of the mix, and dissolve the sugar. Check if it is ready by rubbing a bit of the mix between your fingers. If you can't feel the sugar crystals, it's ready. If you can, beat a little longer.
Spoon the filling over the chilled base, smooth the top and pop in the freezer while you make the next layer.
Clean and dry your mixing bowl.
This time add the blueberries, and the remainder of the ingredients. (ie. same as in step 5). Repeat step 5 beating until you've got great volume out of the mix, and the sugar is dissolved.
Spoon over the chilled base and strawberry layer, smooth the top, and cover with a sheet of baking paper. Freeze for at least 4 hours. (This can keep in the freezer in an airtight container for up to a month.)
To serve, cut cake into wedges using a knife that has been warmed in hot water. Garnish with fresh strawberries and blueberries.
