Preheat the oven to 350° Fahrenheit. Grease a 9x13inch pan with butter or cooking spray. Separate the egg whites and yolks into two clean, dry, large bowls. Using an electric mixer, beat the egg whites until soft peaks form. Add in 1/2 cup of the sugar and continue to beat until stiff peaks form, then set aside. In the egg yolk bowl, add in the remaining 1/2 cup of sugar and the softened butter and beat until lighter and fluffy. Add in the vanilla, matcha powder, flour, starch, baking powder, and salt and stir together with a spatula until fully combined and smooth.
Using the spatula, add 1/3 of the egg whites into the yolk bowl and fold in until combined. Continue folding in 1/3 of the egg whites until entire mixture is combined. Carefully pour batter into prepared pan and smooth to the edges.
Bake for 20-25 minutes or until springy to the touch and knife inserted in the center comes out clean. Once baked, remove and let cool completely.
While the cake is cooling, make the sauce! Combine all sauce ingredients in a bowl and stir until combined. The matcha will not dissolve completely but that's okay! Once the cake is cool, poke holes every inch or so in the cake with a skewer or toothpick, then pour entire sauce mixture over the cake (it will be submerged but once it chills overnight it will have soaked it up entirely!).
Cover the cake and chill it in the fridge for at least 10-12 hours/overnight. The cake will absorb all of the sauce.
Once the cake is chilled, remove from fridge. Make the topping. In a large bowl, using the electric mixer, beat the heavy cream until soft peaks form, then add the powdered sugar to taste and continue to beat until stiff peaks form. Spread whipped cream evenly over the top of the chilled cake and then finish off with the extra matcha powder for dusting (optional but makes a great presentation). Then serve! Always good with a big of extra sauce which you can spoon out of the bottom of the pan once you serve the first piece.