Dissolve the yeast in a bowl with half of the milk. Let stand until bubbly.
Place flour, sugar, vanilla and oil in the bowl of a food mixer. Attach the dough hook; add the yeast and start mixing, adding the remaining warm milk in a thin stream. Mix for 5 to 7 minutes to make a soft and smooth dough. When ready, the dough will wrap around the hook.
Remove from the bowl, quickly stretch the dough and shape into a ball, then place back in the bowl. Cover with cling wrap and place in the cold oven with the light on. Let rise for 1 hour. Gently punch down the dough and let rise for 1 hour longer.
Preheat the oven to 250 C. Line a 23X33cm baking tray with baking paper and sprinkle with some cornflour.
Roll the dough out into a large rectangle; transfer onto the baking tray. Spread the mascarpone evenly on the dough and scatter the marshmallows on top.
Bake in the preheated oven for 7 minutes. Remove from the oven, scatter chocolate chips on top and then bake for 7 to 8 minutes.
Remove from the oven and allow to cool. Top with fresh raspberries and mint leaves. Cut into squares and serve.