1. Preheat the oven to 350°F. Grease and flour a nine-inch round baking pan.
2. Separate the whole egg and place the yolk in one medium bowl and four whites into another. Into the bowl with the egg yolk, add the oil, soy milk, sugar, flour, baking powder, and vanilla. Beat with a hand-held or stand electric mixer on medium-high speed or whisk by hand for one minute.
3. With an electric mixer on high speed, beat the egg whites with the cream of tartar until stiff peaks form. Use a silicone spatula to fold half the whites into the batter and, when almost mixed in, add the rest of the whites and mix until combined and you don’t see any more egg white clumps.
4. Place in the prepared baking pan and bake for 35 minutes, or until a skewer inserted into the cake comes out clean. Let cool for 10 minutes, and then turn the cake out of the pan onto a rack and let it cool completely.
5. When the cake is cool, take the sorbet pints out of the freezer and let sit for 15 minutes to soften. You can also place the sorbet containers into the microwave for 15 seconds or until you can squeeze the container. Don’t let it melt.
6. Slice the cake across into three pieces so that you will have three layers. Place the bottom slice on a serving plate. Place the 1 cup of mango sorbet in the center of the cake. Use a spatula to spread it out to the sides as evenly as you can, moving the spatula back and forth. Place the middle slice of cake on top of the mango sorbet. Place the cup of raspberry sorbet in the center of that cake and spread an even layer. Place the third, top piece of cake, on top.
7. Place in the freezer for eight hours or overnight and keep in the freezer until you serve the cake. It can survive outside the freezer no longer than 30 minutes before it will start melting.
8. Make the mango and strawberry sauces by putting the ingredients for each sauce separately into a food processor and process until smooth. Add more water if too thick. You may strain the strawberry sauce, if desired.