Cover a work surface with a sheet of parchment.
In a large bowl, toss together the rice krispies, strawberries, raspberries, and blueberries.
In a large pot, melt the butter over medium heat. Once it foams and brown specks begin to form, reduce heat to low and immediately stir in the salt, mini marshmallows, and vanilla extract until the marshmallows are melted and smooth.
Remove from heat and immediately stir in the rice krispies and berries until well-coated. Pour the mixture onto the piece of parchment and pat down gently (form into a square, but don't press down too hard), using gloves or another piece of parchment if too hot to handle, into a square about 1 inch thick. If cutting out shapes, use the star or other desired cutter to cut shapes out of the square, oiling the cookie cutter as needed to prevent the marshmallows from sticking. Work quickly and press the excess back into a square and cut out 2-3 more.
Allow to rest at room temperature until set. If desired, dip the rice krispies treats in melted white chocolate or drizzle over the top. You can also sprinkle ground freeze-dried berries or sprinkles. Serve once the chocolate has set or store in an air-tight container for up to 2 days.