Preheat oven to 350 (175C) degrees, with the oven racks arranged in the center. Place liners in a 12-cup muffin pan, or spray with non-stick spray and lightly dust with flour.
Mix the flour, sugar, baking powder and salt in an electric mixer on low speed with the paddle attachment.
Add the cubes of butter and increase the speed to medium. Mix until the mixture resembles coarse sand.
Add the egg whites, whole egg, buttermilk and extracts. Continue mixing until the batter is smooth and creamy, about 1 minute, stopping to scrape the sides of the bowl once or twice.
Portion the batter evenly into the muffins cups so that they are 2/3 full (approximately 3 tablespoon batter per cup)
Bake 20-25 minutes, until the tops spring back when lightly pressed.
Transfer the pan to a wire rack and cool 10 minutes. Remove the cupcakes from the pan and cool completely.
Use a pastry tip or a small paring knife to cut a small hole in the center of each cupcake (save the interior pieces to snack on). Spoon about a heaping teaspoon of the filling into each cupcake (leave any accumulated juices in the bowl), smoothing the tops. Spread about 2 tablespoons frosting on top of each cupcake.
To make the filling:
Lightly crush the raspberries in a small bowl with sugar. I prefer them less sweet, so 1 teaspoon is the right amount for my taste. Add more or less sugar as you like.
To make the frosting:
Beat the butter with the paddle attachment in a stand mixer until fluffy.
Add the sugar and salt and beat until smooth and incorporated. Add 1 tablespoon of the buttermilk and beat 1 more minute. Add the remaining tablespoon if it doesn't look smooth and spreadable.
Refrigerate until ready to frost the cupcakes.
Refrigerate the filled and frosted cupcakes up to 8 hours.
Bake the cupcakes up to 3 days ahead of serving. Keep them in a covered container at room temperature. Fill and frost when ready to serve.
Buttercream can be made ahead. It will keep well for up to 3 days in the refrigerator.