Preheat oven to 325 degrees. Line the bottom of 3, 8" round cake pans with parchment paper. Butter and flour sides of pan (I use Baking PAM to do this).

Use stand mixer with paddle attachment to beat butter and sugar until lightened, about 2 minutes.

Add eggs, one at a time, mixing until well combined, and scraping down sides as needed.

Mix in lemon juice and lemon zest.

In a small bowl, stir together flour, cornstarch, salt, baking soda, and baking powder. Gradually mix into butter mixture, about 1/2 cup at a time, until well combined. Scrape down sides as needed.

Fold in sour cream until well incorporated.

Divide mixture evenly among the three prepared pans. Bake for 20-23 minutes, or until tops of cake are starting to turn golden and a toothpick inserted into center of cake comes out with just a few crumbs.

Let cool in pans for at least 5 minutes before transferring to cooling rack. Cool completely before assembling.

In a small pot over medium-high heat, add cranberries, sugar, and lemon juice. Bring to a gentle boil. Stir constantly, using your spoon to smash the cranberries as you stir. Continue to gently boil until mixture has thickened, about 8-10 minutes.

Remove from heat. Transfer filling to a heat safe sieve over a heat safe bowl. Use your spoon to press liquid through sieve.

In base of a stand mixer, beat butter and shortening until lightened and fluffy, about 3 minutes.

Mix in salt and vanilla extract.

Gradually beat in powder sugar, about 1/2 cup at a time. After 1 cup of powder sugar is mixed in, add 1 tbsp milk and mix well. Repeat until desired consistency and sweetness is reached (for me this is 4 cups of powdered sugar and 4 tbsp milk).

Place white chocolate in a microwave safe bowl. Heat at 50% power in 30 second increments, stirring after each increment.

Turn mixer on low speed and slowly stream in the white chocolate. Once all the chocolate is in, turn to medium-high speed and beat for about 1 minute.

Level each layer of the cake. Use frosting to make a barrier around the outside of two of the cake layers. Divide the cranberry mixture between the two barrier layers. Stack the filled layers on top of each other. Place the final layer leveled side down.

Cover cake in a thin layer of white chocolate frosting. Place in refrigerator for 10 minutes. Remove and continue to frost as desired.