Heat grill to a medium-low heat and lightly butter 1 side of each slice of cake.
Place cake butter side down on a plate or wax paper and evenly spread cream cheese on one slice and mascarpone cheese on the other. Top cream cheese with graham crackers and blueberry pie filling. Create a layer cake by adding the second slice of angel food cake, cheese side down, on top of the blueberry pie filling. Press together gently to seal.
Carefully transfer the cake onto the grill and grill for 1 to 2 minutes, until the cake starts to brown. Carefully flip the cake to grill the other side. Remove from the heat and garnish with powdered sugar, whipped cream, and fresh blueberries.