1 package (12-ounce) semi-sweet chocolate chips
½ cup butter
2 cups graham cracker crumbs
1 package (8-ounce) cream cheese
1 can (14-ounce) sweetened condensed milk
1 egg
1 teaspoon vanilla extract

Preheat oven to 325 F. To prepare crust: In medium saucepan over very low heat, melt chocolate chips and butter, stirring until smooth. Stir in graham cracker crumbs. Remove ¼-cup mixture for later use. Press remaining graham cracker mixture evenly in a 9-x13-inch baking pan.
In a large bowl with an electric mixer, beat cream cheese until smooth. Gradually beat in condensed milk, then egg and vanilla. Pour over prepared crust. Sprinkle with reserved ¼ cup crumb mixture. Bake until set, 25-to-30 minutes. Cool. Refrigerate until cold, about 2 hours or longer. Cut into 24 squares.