01 - Preheat oven to 325°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a medium bowl, combine graham cracker crumbs, sugar, and melted butter. Mix until evenly moistened. Press firmly into the bottom of the prepared pan. Bake for 8 minutes. Let cool slightly.
03 - In a large bowl, beat cream cheese until smooth. Add sugar and mix until creamy. Beat in eggs, one at a time. Stir in sour cream, vanilla, and salt until just combined. Pour over the cooled crust and spread evenly.
04 - In a small saucepan, combine strawberries, sugar, lemon juice, and cornstarch. Cook over medium heat, stirring, until strawberries break down and the mixture thickens, about 5 minutes. Let cool slightly, then dollop over the cheesecake layer. Swirl gently with a knife for a marbled effect.
05 - Bake for 25-27 minutes, until the center is just set. Cool to room temperature, then refrigerate for at least 4 hours or until fully chilled.
06 - Melt chocolate and butter together in a heatproof bowl set over simmering water or in the microwave in 20-second bursts. Stir until smooth. Pour over chilled cheesecake, spreading evenly. Refrigerate until chocolate is set, about 30 minutes.
07 - Lift bars from the pan using the parchment overhang. Cut into 16 squares. Serve chilled.