1. Preheat oven to 400°F. Wash hands with soap and water. Lightly spray the inside of a 9-inch springform pan with cooking spray.  
2. To make crust, mix the graham crackers crumbs with melted margarine in a medium bowl. Press into the bottom of the springform pan. Bake 4-5 minutes, or until crust is lightly brown. Reduce oven temperature to 325°F. 
3. To make the filling, mix the cream cheese, Splenda®, and flour together in a large bowl with a spoon or fork until blended. Add eggs, vanilla and mix until smooth, add the milk while mixing. Mix until smooth.
4. Transfer ½ cup of the batter to a small bowl. Add the dulce de leche and whisk until combined. Pour the plain batter into the crust. Add dulce de leche by the spoonful to the top of the batter. Insert a butter knife into the batter and swirl the two batters together for a marbling effect.
5. Bake until the sides of the filling are slightly puffed, and the center is almost set, 45 to 55 minutes. Cool on a wire cake rack. Cover with plastic wrap and refrigerate until chilled, at least 8 hours or overnight. 

Store in an airtight container and refrigerate leftovers for up to seven days.