Position a rack in the center of the oven and preheat to 400°F. Lightly spray the inside of a 9-inch spring form pan with the cooking spray.
To make the crust, mix the crumbs and melted margarine in a medium bowl until the crumbs are evenly moistened. Press the crumb mixture evenly into the bottom of the prepared pan.
Bake until the crust is lightly browned, about 7 minutes. Reduce the oven temperature to 325°F.
To make the filling, mix the cream cheese, Splenda, and flour together in a large bowl with a spoon or fork until blended. Add vanilla and mix on medium until smooth, adding the milk while mixing. Mix until smooth.
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 spoonfuls to top of the batter. Insert a butter knife into the batter and swirl the two batters together for a marbling effect.
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
Serving Size 1 Pie
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.