Make your cake! You'll be cutting it into four layers later. Spray a 9x9 inch square cake pan with cooking spray and line with parchment paper, then spray again and set aside. In a large bowl, whisk the eggs, sugar, oil, sour cream, milk, and vanilla until smooth. Add in the flours, starches, black sesame powder and paste, and baking powder and soda, and salt, and whisk thoroughly until fully mixed and smooth. If the batter feels too thick (it should be easily pourable) add 1-2 more tablespoons of milk. Then pour batter into the prepared pan and baked for 35-45 minutes or until a skewer or knife inserted into the center of the cake comes out clean. It will dome nicely and have a hard crust on top. Let cake cool completely before turning it out onto a cooling rack, then carefully slice off the domed top so you're left with a perfectly flat top. Then slice the cake in half giving you two thick rectangles, and then carefully slice each of the rectangles into two layers to get four equal-sized rectangular cake layers. Continue to cool before constructing.