Add 1 tablespoon lemon juice to an electric blender along with 5 cups fresh sliced strawberries and ¼ cup white sugar. Blitz to combine, until smooth. Transfer this mixture to a saucepan and simmer for about 20 to 25 minutes, until reduced by half, stirring frequently.
Once done, turn off the heat and let it cool for 15 to 20 minutes.
Reserve ½ cup plus 5 tablespoons for frosting and remainder for cake filling.
Take two cake pans (9-inch), grease, and line with parchment paper.
Whisk together 1 teaspoon baking soda, 1 teaspoon baking powder, 3 cups all-purpose flour, and ½ teaspoon salt in a medium bowl. Keep aside.
Beat together 2 cups white sugar and 1 cup unsalted butter using an electric mixer until creamy.
Start beating in eggs, one by one, missing after each addition. Mix in 1 teaspoon vanilla extract and 1 ½ tablespoons lemon zest. Stir in ½ of the lemon juice, ½ of the sour cream, and ⅓ of the flour mixture until well combined.
Repeat alternating additions of lemon juice, sour cream, and flour until just combined. (Avoid over mixing)
Transfer half of the batter in a separate bowl and fold in ½ cup strawberry reduction. Now start placing dollops of both strawberry-lemon and lemon batters into the prepared cake pans alternately. Swirl the batters with a knife together.
Bake in a preheated oven at 350 F for 30 to 35 minutes, until a toothpick inserted comes out clean from the center. Once done, take them out and allow them to cool for 10 minutes. Then cool for another 30 minutes on a wire rack.
Meantime, beat 1 ½ cups unsalted butter in a bowl by using an electric mixer until light and fluffy. Start beating in sugar, one cup at a time, until well combined.
Mix in 1 teaspoon lemon zest, 1 tablespoon lemon juice, ¼ teaspoon salt, 1 teaspoon vanilla extract, and 5 tablespoons strawberry reduction until well combined. Beat in 4 tablespoons heavy cream until fluffy and smooth, for a few minutes. If it is too thin, add additional sugar, or if it is too thick, add more heavy cream.
Place one cake on a serving platter and top with a thin frosting layer.
Fill the piping bag with frosting pipe a border around the cake edge, leaving the center open.
Pour strawberry reduction in the center. Top with second cake and continue frosting as desired.
Refrigerate until ready to serve.