Preheat your oven to 350°F and line a cupcake pan with paper liners. While the oven heats, whisk together the flour, cornstarch, baking powder, salt, and cream of tartar in a medium bowl—this ensures even distribution of leavening agents. I like to sift the dry ingredients together twice to eliminate any lumps that could create dense spots in the cupcakes. Set this mixture aside and gather all other ingredients to room temperature.

While the oven preheats, puree the fresh strawberries and strain through a fine-mesh sieve to remove seeds, collecting about 3/4 cup of liquid. Pour this liquid into a small saucepan and bring to a gentle boil over medium-high heat. Let it reduce for 15-20 minutes, stirring occasionally, until you have about 1/4 cup of concentrated strawberry liquid with deep, intense flavor. Transfer to a small bowl and let cool completely—this can be done while you bake the cupcakes.

In a large mixing bowl, beat together the room-temperature butter, granulated sugar, and freshly grated lemon zest on medium speed for 2-3 minutes until light, fluffy, and pale. This creaming process incorporates air that helps the cupcakes rise and creates a tender crumb. Add the egg whites one at a time, beating well after each addition, then add the vanilla extract and mix until fully combined.

With the mixer on low speed, alternate adding the dry ingredient mixture from Step 1 and the sour cream mixed with lemon juice, starting and ending with the dry mixture. Add in three additions: dry, then sour cream mixture, then dry, then sour cream mixture, and finally dry. This prevents overmixing and keeps the batter light and tender. Mix just until combined—I recommend using a rubber spatula to fold the batter gently by hand after the last few additions to ensure you don't overdevelop the gluten.

Divide the batter evenly among the lined cupcake cups, filling each about 2/3 full. Place in the preheated 350°F oven and bake for 15-18 minutes, until a toothpick inserted into the center comes out clean or with just a few moist crumbs. The cupcakes should be light golden and spring back when lightly touched. Allow to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before frosting.

In a clean bowl, beat the unsalted butter on medium speed for 1-2 minutes until creamy. Add 1 1/2 cups of the sifted powdered sugar and beat until smooth and fluffy, then add the cooled strawberry reduction from Step 2 along with the strawberry extract. Continue beating, then gradually add the remaining powdered sugar and heavy whipping cream, beating until you reach a spreadable consistency with a beautiful pale pink color. Taste and adjust sweetness if needed—the frosting should balance the tartness of the lemon cupcakes with bright strawberry flavor.

Once the cupcakes are completely cooled, use an offset spatula or piping bag to apply the strawberry-lemon frosting generously to the top of each cupcake. For an elegant presentation, I like to create a slight swirl with the back of a spoon or top with a fresh strawberry slice if desired. Serve immediately or store in an airtight container at room temperature for up to 2 days.