Preheat the oven to 350°F with a rack in the center. Line two standard cupcake pans with paper liners.
In a large bowl, sift together the flour, baking powder and salt.
In a measuring cup, combine the buttermilk, vanilla, and lemon extract.
Rub sugar and lemon zest together until the sugar is moistened and fragrant. In the bowl of a stand mixer fitted with the paddle attachment, or with a hand mixer, beat the butter and sugar at medium speed for 3 minutes, or until the butter and sugar are very light and fluffy. Add 1/3 of the flour mixture, beating on low speed. Beat in half of the milk mixture, then beat in half of the remaining dry ingredients until well incorporated. Add the rest of the milk, beating until the batter is fully incorporated and smooth, then add the last of the dry ingredients, beating until the ingredients are thoroughly combined.
Add the egg whites to a medium bowl and beat with a hand mixer on medium-high speed until stiff peaks form.
Gently stir ¼ of the whipped egg whites into the cupcake batter with a rubber spatula until no streaks of egg whites remain. This will help lighten up the batter. Gently fold in the remaining whipped egg whites, one half at a time, by cutting the spatula down the middle to the bottom of the bowl, then sweeping it around, up and over. Turn the bowl 90 degrees and repeat until no streaks of egg whites remain.
Using a large scoop (about 3 tablespoons), distribute the batter between 24 muffin wells, filling each well 2/3 full.
Bake for 18-22 minutes, or until the cupcakes are springy to the touch and a toothpick inserted into the centers come out clean. Cool on wire rack.
Once cupcakes have cooled, hollow out a small circle in the center of each cupcake. Add a spoonful of lemon curd into each hollow. Frost with lemon blueberry buttercream or as desired.
For the Lemon-Blueberry Frosting
In a large mixing bowl, cream butter until fluffy. Slowly add in confectioner’s sugar, and continue creaming until well blended.
Add salt, vanilla, lemon zest and blueberry puree and mix until well combined. If buttercream is too thick, slowly drizzle in milk until you reach the desired consistency. Beat at high speed until frosting is smooth and fluffy.