Melt the butter in a large pot over low heat. Add the marshmallows, stirring until completely melted. Add the cereal and stir to coat evenly.
Transfer the ooey-gooey mixture to a parchment lined springform pan. Press into the pan and allow to cool for at least 60 minutes. Meanwhile, melt the white chocolate and mascarpone on very low heat. Once liquified-ish, pour the chocolate cheese mixture over the cooled cereal fluff cake layer.
Allow the cake to cool again – in the fridge for several hours.
In the meantime, whip together the cream cheese and strawberry preserves.
Once the cake is cool, spread a thin layer of the cream cheese over the top. Then decorate with freshly sliced strawberries.