Make the streusel by mixing well all of the streusel ingredients in a bowl. Set aside.
Whisk together the sugar, oil and vanilla extract for a minute, either in the bowl of a stand mixer or with a hand mixer. If you don't have either, use a whisk but make sure you beat well for about 2 minutes.
Add the nondairy milk, mashed bananas and applesauce and beat for another minute until everything is well incorporated.
Mix the flour with the baking soda and baking powder. Whisk.
Add the flour to the banana-oil-applesauce mix in two batches, beating for 20 seconds after each addition. Scrape the sides and bottom of the bowl with a spatula to ensure everything's well integrated and you don't have any streaks of dry flour.
Oil and flour a cake pan. The one I used was a square pan that's 9 X 9 X 2 inches, but you can use a regular 8- or 9-inch cake pan if you'd rather.
Pour half of the cake batter in the pan and with a spatula make sure it is spread in an even layer. Sprinkle half of the chocolate streusel topping on the batter.
Pour in the remaining half of the batter and smooth over the chocolate streusel. Sprinkle the remaining streusel on top.
Bake the cake in a preheated 350-degree Fahrenheit oven for 45 minutes or until a toothpick inserted in the center comes out clean.
Cool in the pan on a rack.
Slice and eat.