1) Preheat the oven to 350°F. Lightly grease and flour (or line with parchment) two 8" square pans, or two 9" round pans.
2) To make the cake: Beat the butter, sugar, and oil until the mixture lightens in color and texture, 2 to 3 minutes at medium speed.
3) Beat in the baking powder, salt, cake enhancer, and vanilla extract.
4) Add the egg whites and the whole egg, beating until the mixture is fluffy. Scrape the bottom and sides of the bowl.
5) Beat in the flour in three additions, alternating with the yogurt or milk. Scrape the bowl after each addition; the mixture should be fluffy when done.
6) Scoop the batter into the prepared pans. Wrap the pans in cake strips, if you have them, to prevent doming.
7) Bake the cakes for 32 to 37 minutes, until a cake tester or toothpick inserted into the center comes out clean.
8) Cool the cake in the pans for 10 minutes, then turn them onto a rack to cool completely.
9) To make the filling: Bring the water to a boil in a small saucepan set over medium heat.
10) Add the lemon pastry filling mix and whisk vigorously to blend as the mixture comes back to a boil.
11) Remove from the heat and whisk in the butter, stirring until it's completely melted.
12) Transfer to a bowl and chill in the refrigerator until firm, 3 hours to overnight. Stir the lemon filling until spreadable.
13) To assemble the cake: Gently stir the filling to loosen it, then spread 1/2 cup of filling atop each cake layer; don't stack them yet. You'll have some filling left over.
14) Add 1/2 cup cream to the remaining filling, beating until smooth. Add the remaining 1 ½ cups cream, beating until light and fluffy.
15) Spread a third of this lightened filling atop one layer. Place the second layer on top; spread the top layer with the rest of the lightened filling. Chill for 2 hours before serving.
Yield: 16 servings.