Butter a 9 by 13 inch pan and set it aside.
Brown the butter. Melt the butter in a large pot over medium heat, swirling often, until it foams and the milk solids at the bottom turn golden and smell nutty. Watch closely at the end so it does not burn.
Reduce the heat to low. Add the 320 g marshmallows and the salt. Stir until completely melted, smooth, and glossy, then pull the pot from the heat the moment it is smooth.
Add the rice crisps and fold until they start to become coated.
Fold in the 1 cup broken strawberries and the 1 cup whole blueberries, then add the 2 tablespoons crushed strawberries and 2 tablespoons crushed blueberries and fold again so the color runs all the way through.
Fold in the reserved 100 g marshmallows so they stay mostly whole and create soft pockets.
Transfer to the prepared pan and press into an even layer with buttered hands or a sheet of parchment. For more color on top, sprinkle on the optional crushed fruit and press it in.
Let set at room temperature for 30 to 60 minutes, then cut into bars.
For a finished look, drizzle the cut bars with melted white chocolate and dust the wet drizzle with pulverized dried strawberries. Let the drizzle set before serving.