Step 1: For the Strawberry Puree: Hull and finely dice 1 cup of fresh strawberries. Place them in a small saucepan over medium-low heat. Add 1 tablespoon of granulated sugar and 1 teaspoon of lemon juice. Cook, stirring occasionally, until the strawberries break down and the liquid reduces, about 10-12 minutes. The mixture should be thick and jam-like. Mash any remaining chunks. Let cool completely.
Step 2: For the Cupcakes: Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.
Step 3: In a medium bowl, whisk together 1 ½ cups cake flour, 1 teaspoon baking powder, ½ teaspoon baking soda, and ¼ teaspoon salt. Sift the dry ingredients twice.
Step 4: In a large bowl, cream together ½ cup (1 stick) unsalted butter, softened, and 1 cup granulated sugar until light and fluffy, about 3-5 minutes. Scrape down the sides of the bowl as needed.
Step 5: Beat in 2 large eggs, one at a time, ensuring each egg is fully incorporated before adding the next. Add 1 teaspoon vanilla extract. Mix until just combined.
Step 6: In a separate small bowl, whisk together ½ cup buttermilk and the cooled strawberry puree from Step 1.
Step 7: Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the buttermilk mixture, beginning and ending with the dry ingredients. Mix on low speed until just combined after each addition. Do not overmix.
Step 8: Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full.
Step 9: Bake for 18-22 minutes, or until a wooden skewer inserted into the center of a cupcake comes out clean. Let the cupcakes cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely.
Step 10: For the Strawberry Buttercream Frosting: In a small saucepan, combine 1 cup fresh, hulled and diced strawberries with 1 tablespoon granulated sugar. Cook over medium-low heat, stirring frequently, until the strawberries break down and the mixture thickens into a jam-like consistency (10-15 minutes). Mash any remaining chunks. Let cool completely, then chill in the refrigerator for at least 30 minutes to thicken further.
Step 11: In a large bowl, beat 1 cup (2 sticks) unsalted butter, softened, with an electric mixer on medium speed until creamy and pale, about 3-4 minutes. Scrape down the sides of the bowl.
Step 12: Gradually add 3-4 cups powdered sugar, one cup at a time, beating on low speed until combined, then increasing to medium-high until fluffy. Add 1 teaspoon vanilla extract. If the frosting is too thick, add milk or cream, ½ teaspoon at a time, until desired consistency is reached.
Step 13: Once the strawberry reduction from Step 10 is thoroughly chilled and thick, gradually add it to the buttercream, beating on low speed until just incorporated. Increase speed to medium-high and beat for another 1-2 minutes until the frosting is light, fluffy, and beautifully pink.
Step 14: Once the cupcakes are completely cool, pipe or spread the strawberry buttercream onto each cupcake. Garnish with a fresh strawberry slice or a small whole strawberry, if desired. Enjoy!