Preheat oven to 350F. Line a 12-count cupcake pan with cupcake liners; set aside.

In a large mixing bowl, cream together the softened butter and sugar with an electric mixer until light and fluffy. Add the egg whites and vanilla extract; mix until well combined. Add the milk.

In a separate bowl, combine the dry ingredients: flour, salt and baking powder. Using a sifter, add the dry ingredients gradually into the batter, mixing after each addition. Be careful not to over mix, beat only until the flour is incorporated.

Fill the cupcake liners two-thirds of the way full. Bake the cupcakes in preheated oven for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean. Remove the cupcakes from the pan and set onto cooling rack to cool completely before frosting.

Once cooled, prepare the frosting. Place the softened butter into a mixer bowl and whisk on high speed for 8 to 10 minutes until butter is light and fluffy. Scrap down the sides of the bowl often. Add the confectioner's sugar gradually and mix after each addition. Once all the sugar is added, whisk for about 5 minutes until frosting is fluffy. Add the raspberry extract, red food coloring and raspberries last. Mix for about 30 seconds. Transfer the frosting into a pastry bag for piping.

Fill the cupcakes with lemon curd: using a melon baller or small measuring spoon, scoop out a small center on each cupcake. Transfer the lemon curd into a pastry bag tipped with a Wilton 230 tip or similar. Fill the center of each cupcake with generous amount of lemon curd.

Frost each cupcake with frosting, then garnish with fresh raspberries and mint leaves, if desired. Store in refrigerator if not serving immediately; remove 1 hour prior to serving to allow frosting to soften.