Preheat oven to 325 degrees. Line three, 8″ round pans with parchment paper. Flour and butter sides of pan (I use baking PAM to do this).
In large mixing bowl, whisk together flour, cornstarch, salt, baking soda and baking powder.
In base of a stand mixer, whisk together sugar and lemon zest. Add softened butter and beat on medium-high speed until lightened and fluffy, about 1.5 minutes. Then, beat in eggs, one at a time, scraping down sides as needed.
Stir together oil and lemon juice in a small bowl. Then, alternate mixing in flour mixture and oil mixture in about 3 additions each. After each addition, start on low speed and gradually increase to medium-high for a few seconds. Once all flour and oil has been added, beat on medium-high until well combined. Finally, stir in sour cream until just combined.
Divide batter among the three prepared pans and bake for 27-30 minutes, or until tops are golden and toothpick in center of cake comes out with just a few crumbs. Do not open oven during baking time.
Let cakes cool in pans for at least 10 minutes before removing. Cool completely before frosting.
Strawberry filling
Put thawed strawberries in blender and puree until smooth. This will make about 1 3/4 cups of strawberry puree. Place 3/4 cup of the puree in a small sauce pan to use for filling. Set remaining puree aside until preparing frosting.
Add sugar, lemon juice and cornstarch to puree in sauce pan.
Heat over medium heat, whisking constantly, until sauce thickens, about 10-15 minutes. Remove from heat.
Transfer mixture to a heat safe container with lid, and place in fridge for at least 2 hours.
Frosting directions
In large mixing bowl, beat together butter and shortening on medium-high until light and fluffy, about 2 minutes.
Beat in 1/2 cup powdered sugar, followed by 1 tablespoon cooled strawberry puree. Then beat in salt and vanilla. After this, add 1/2 powdered sugar and 1 tbsp strawberry puree at a time until desired consistency and strawberry taste is reached. NOTE: You will likely not use all of the powdered sugar or all of the puree (I use about 4 cups powdered sugar and about 1/2 cup of strawberry puree). Be careful when watching the consistency, if you put in too much of the puree, your frosting will become too liquid to stick well on the cake.
Assembly
Level cakes and place one layer on a cardboard round.
Use frosting bag with large, round opening to pipe border around outside of cake. For this I use a #12 Wilton tip.
Cake layer piping
Fill inside of layer with half of cooled filling mixture.
Strawberry filling
Layer on second cake round and repeat. Place third layer on top.
Coat outside of cake with thin layer of frosting. Place cake in fridge for about 15-20 minutes.
crumb coating
Remove cake and finish frosting. I used the #12 tip to pipe the bottom of the cake and the frosting decor on top. Decorate with strawberries and lemon peel as desired.
