Slice the brioche into six ½" thick pieces.
Spread the mascarpone cheese onto one slice of the bread, and add the preserves to the other.
Place a few fresh slices of strawberries on the side with the mascarpone cheese.
Make a sandwich with both of the slices of bread and press the mascarpone and strawberry together, then set aside.
In a large mixing bowl, whisk together the egg, milk, vanilla, and cinnamon.
Take the french toast sandwich and dip into the egg and milk mixture, holding in the mixture for a few seconds each side. In a frying pan over medium-low heat with melted butter, add the stuffed french toast. Let the sides get golden brown, about 2 minutes then flip.
When done cooking, add to a plate and dust with powdered sugar to coat, then serve.