Whisk in a bowl the mascarpone on medium speed (about 1 minute) until smooth
Add heavy cream, powdered sugar, vanilla extract and salt
Whip until stiff peaks (3-4 minutes)
Place slices of shokupan on a clean cutting board and add 3 tablespoons of whipped cream mixture on to each slice of bread
To build one sandwich, place 4-5 strawberries on their sides onto one slice of bread in an "X" pattern that runs from corner to corner
Cover with a second slice of bread, cream side down
Press gently down, be sure that the cream does not get out of the sides
Wrap sandwich in plastic wrap
Refrigerate until cream has chilled and sandwich is firm when pressed gently, about 1 hour
When ready to serve, remove the plastic wrap and place sandwiches on cutting board
Remove crusts carefully with a sharp knife
Cut sandwiches in half diagonally, slicing the strawberries in half
Serve immediately