In a blender, puree the raspberries and strawberries until completely smooth. Place a fine mesh sieve over a bowl and pour the puree into it to strain out the seeds. Set aside

In a medium bowl, whisk the flour, salt, and baking powder together and set aside. In the bowl of stand mixer fit with a paddle, on medium speed, beat the butter and sugar together until smooth. Add the eggs, one at a time until well mixed. Beat in the milk, strawberry puree, and vanilla extract until combined.

Add the reserved flour mixture to the wet ingredients and mix on low for 1-2 minutes, until the wet and dry ingredients are just combined.

Pour the batter into the prepared muffin pan, distributing the batter evenly. Bake for 15-17 minutes or until a toothpick comes out clean. Remove pan from the oven and let cool for 10 minutes. Remove the cupcakes from the pan and continue cooling on a cooling rack.

To make the frosting, whisk the butter in a bowl until smooth. Then slowly whisk the powdered sugar into the butter until combined and fluffy. Add 1 tablespoon raspberry strawberry puree into the frosting until just combined. Add additional puree if you would like a smoother frosting.

Pipe or spread the frosting on the cupcakes when they are completely cool.