In a medium-sized mixing bowl, stir together mascarpone cheese, diced strawberries, powdered sugar, and vanilla bean paste.
Lay out the challah bread slices on a clean surface. Spread 1 tablespoon of butter on one side of each slice.
Flip two slices over and spread half of the mascarpone mixture on each slice of bread.
Top each slice with another slice of bread, butter side up.
Add the sandwiches to a large skillet or flat griddle pan set over medium heat.
Cook until the underside is golden brown, then carefully flip the sandwiches over.
Continue to cook until the other side is also golden brown.
Transfer the stuffed toast to a large plate.
Top each piece with a dollop of whipped cream and a sprinkle of powdered sugar.