Line an 8-inch (20cm) square pan with parchmente paper or aluminum foil.
In a heavy medium saucepan, combine 1 cup sugar, 1 cup brown sugar, ¾ cup milk and 2 tablespoons corn syrup.
Add 2 ounces bittersweet chocolate and cook over medium heat, stirring constantly, until the mixture boils. Attach a candy thermometer and let it boil slowly, stirring occasionally, until it reaches 236ºF/113°C, or soft ball stage.
Remove from the heat, add 3 tablespoons unsalted butter on top, but don't stir. Let cool to lukewarm (110ºF/43°C), like warm tap water.
Add pinch salt, 1 teaspoon vanilla extract, ⅓ cup peanut butter and mix well. Add ¾ cup mini marshmallows and pour into the prepared pan, spreading evenly.
Put in the fridge until cold. Cut into squares and serve.
Keep refrigerated, well wrapped.