Preheat the oven to 350 degrees. Line a cupcake pan and set aside.
In the bowl of a stand mixer, combine the butter, sugar, salt, and baking powder. Cream until smooth.
Add in the milk and eggs and mix well.
Add the flour and mix on medium speed for two minutes.
Fill each cupcake liner until about ⅔ full. Bake in preheated oven for approximately 14-16 minutes or until edges are slightly browned and a toothpick comes out clean. Cool completely.
Filling
Cook the berries over low in a small saucepan until warm and juicy. Press the mixture through a fine mesh sieve.
Reserve two tablespoons of the puree to use in the frosting.
Transfer back to the saucepan.
Combine the cornstarch with one tablespoon of water. Stir the cornstarch and sugar into the warm berry mixture. Heat over low until slightly thickened.
Frosting
In the bowl of a stand mixer, cream the butter until smooth.
Add in the powdered sugar a cup at a time, alternating with the raspberry puree.
Mix on low speed for 30 seconds and then medium speed for about a minute. Stop the machine, scrape down the sides and mix again. If the frosting appears too lumpy and dry, add a tablespoon of milk, only if needed. Mix again.
Assembly
Transfer the frosting to a piping bag fitted with a large star tip.
Make a well in each cupcake. An apple corer works for this if you do not have a cupcake corer.
Put 2 fresh or frozen raspberries in each well. Top with a teaspoon of raspberry sauce.
Pipe a mound of frosting on each cooled cupcake. Garnish with additional fresh berries, if desired.