1. Preheat the oven to 350°F (175°C) and lightly grease and flour two 8-inch square pans or two 9-inch round cake pans.

2. Beat the softened butter, sugar, and vegetable oil for 2 to 3 minutes until light and fluffy.

3. Add the baking powder, salt, and vanilla extract and mix until combined.

4. Add the egg whites and whole egg and beat until the mixture is fluffy, scraping the bowl as needed.

5. Add the cake flour in three additions, alternating with the yogurt, beginning and ending with the flour. Mix just until combined.

6. Divide the batter evenly between the prepared pans and smooth the tops.

7. Bake for 32 to 37 minutes, or until a toothpick inserted into the center comes out clean.

8. Cool the cakes in the pans for 10 minutes, then transfer them to a wire rack and cool completely.

9. Prepare the pastry cream mix with the milk until thick according to the package directions.

10. Stir the lemon curd into the prepared pastry cream until evenly combined.

11. Cover and refrigerate the lemon filling for 2 to 3 hours until firm.

12. Place one cooled cake layer on a serving plate and spread about 1/2 cup of lemon filling over the top.

13. Place the second cake layer on top.

14. Add 1/2 cup of heavy cream to the remaining lemon filling and beat until smooth.

15. Add the remaining heavy cream and beat until light and fluffy.

16. Spread the lemon whipped cream frosting over the top and sides of the cake.

17. Refrigerate the assembled cake for about 2 hours before serving.