Combine the pureed strawberries, 1/2 cup sugar, and 1/4 cup lemon juice in a saucepan over medium heat. Simmer for 20-30 minutes, stirring occasionally, until the mixture reduces by half and becomes deeply concentrated and syrupy. This reduction is the flavor backbone of your cake, so let it cook until it reaches a rich, jammy consistency. Once done, transfer to a bowl and refrigerate while you prepare the cake batter.

Preheat your oven to 350°F and grease three 8-inch or 9-inch round cake pans, then line the bottoms with parchment paper for easy removal. While the oven heats, whisk together the flour, baking powder, baking soda, and salt in a medium bowl—this dry mixture is your foundation for an evenly leavened cake. Measure out the milk and lemon juice (1/4 cup), and have your room-temperature eggs and butter ready. I like to crack my eggs into a small cup first so you can add them individually to the butter mixture without any shell mishaps.

Using an electric mixer, beat the room-temperature butter, 2 cups sugar, lemon zest, and vanilla extract together for 5 minutes until the mixture is pale, fluffy, and noticeably lighter in color. This creaming process incorporates air into the batter, which helps the cake rise and creates a tender crumb. The lemon zest will distribute throughout, ensuring every bite has bright citrus flavor.

Add the room-temperature eggs to the butter mixture one at a time, beating well after each addition to ensure they're fully incorporated and the batter stays emulsified. Reduce your mixer speed to low, then alternate adding the dry ingredient mixture from Step 2 and the milk, starting and ending with the dry mixture. This alternating method prevents overmixing and ensures the flour doesn't develop too much gluten, which would make the cake dense. Finally, mix in the 1/4 cup lemon juice until just combined—I find this brightens the crumb and helps the strawberry flavor really shine through.

Divide the batter evenly among the three prepared pans and smooth the tops with a spatula. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs. The cakes should be golden brown on top and springy to the touch. Let the cakes cool in the pans for 10 minutes, then turn them out onto wire racks to cool completely—this takes about 1-2 hours.

Using an electric mixer, beat the 2 cups room-temperature butter for 5 minutes until it's creamy and pale. Gradually add the sifted powdered sugar, a cup at a time, beating on low speed until combined before adding the next batch. Once all the sugar is incorporated, add the cream of tartar, then drizzle in the chilled strawberry reduction from Step 1, the 3 tbsp lemon juice, and the heavy cream. Beat on medium speed for 1-2 minutes until the frosting is light, fluffy, and has the perfect spreadable consistency. The frosting should be vibrant pink with flecks of lemon zest visible throughout.

Place the first cooled cake layer on a serving plate and spread a thin layer of the strawberry lemonade frosting from Step 6 on top. Add a second layer, then frosting, then the final layer. Frost the top and sides of the entire cake with the remaining frosting, creating a smooth or rustic finish as desired. Transfer the assembled cake to the refrigerator and chill for at least 2 hours before serving—this helps the frosting set and makes slicing cleaner.