In a large mixing bowl, beat the softened butter with an electric mixer on high speed for about 5 minutes until it becomes wonderfully light and fluffy. Don't forget to scrape down the sides of the bowl as needed to ensure even whipping.
Reduce the mixer speed to low. Slowly add the sifted powdered sugar, one spoonful at a time, making sure each addition is fully incorporated before adding the next. Continue beating until the mixture is smooth and fully combined.
Stir in the salt and vanilla extract until they are well mixed into the frosting, enhancing its delicious taste.
Now, slowly pour in the heavy cream or milk, one tablespoon at a time, until your frosting reaches a beautiful, smooth, and easily spreadable consistency. Give it one final minute of whipping on medium speed to make it light and airy.
Arrange your store-bought brownie bites neatly on a serving platter or tray. Use a piping bag (or a simple plastic baggie with a small corner snipped off) to add a lovely swirl or dollop of your homemade buttercream frosting onto each brownie.
Finish your Valentine's Day masterpieces by adorning them with fresh, vibrant berries like strawberries or raspberries. Add a sprinkle of festive Valentine's Day candies or sprinkles for that extra touch of love and color!