Step 1: Prepare the Graham Cracker Crust. In a medium bowl, combine the graham cracker crumbs, ¼ cup granulated sugar, and melted butter. Mix well until the crumbs are evenly moistened. Press the mixture firmly into the bottom of a 9-inch springform pan. Place the crust in the freezer for at least 15-20 minutes while you prepare the fillings.
Step 2: Make the Blueberry Swirl. In a small saucepan, combine the blueberries, ¼ cup granulated sugar, 1 tablespoon lemon juice, and cornstarch (if using frozen blueberries). Cook over medium heat, stirring frequently, until the blueberries burst and the mixture thickens slightly, about 5-7 minutes. Remove from heat and let cool completely.
Step 3: Prepare the Mango Puree. In a food processor or blender, combine the diced mangoes, 1 tablespoon lemon juice, and ½ cup granulated sugar. Blend until completely smooth. Set aside.
Step 4: Create the Ice Cream Base. In a large, very cold mixing bowl, whip the heavy cream with an electric mixer on high speed until stiff peaks form. Gently fold in the cold sweetened condensed milk until just combined.
Step 5: Assemble the Mango Layer. Carefully fold about two-thirds of the ice cream base into the cooled mango puree until just combined. Pour this mango ice cream mixture over the chilled graham cracker crust in the springform pan. Smooth the top and return to the freezer for at least 2 hours, or until firm.
Step 6: Add the Blueberry Layer. Once the mango layer is firm, gently spread the remaining one-third of the plain ice cream base over the mango layer. Drizzle spoonfuls of the cooled blueberry swirl over the white ice cream layer. Using a butter knife or a skewer, gently swirl the blueberry mixture into the white ice cream to create beautiful ribbons.
Step 7: Freeze Until Solid. Cover the springform pan tightly with plastic wrap and freeze for at least 6 hours, or preferably overnight, until the cake is completely solid.
Step 8: Serve and Enjoy! When ready to serve, remove the cake from the freezer and let it sit at room temperature for 10-15 minutes to soften slightly. Run a thin knife around the edge of the pan before releasing the springform collar. Garnish with fresh mango slices, blueberries, and mint if desired. Slice with a warm knife for clean cuts.