Preheat the oven to 350°. Lightly butter a 9-by-13-inch baking dish. Line the dish with parchment paper, leaving 1 inch of overhang on the long sides.
In a small saucepan, melt the chocolate chips in the sweetened condensed milk over low heat, stirring constantly, until smooth and thickened, 3 to 5 minutes. Remove from the heat. Stir in the vanilla and let cool to room temperature.
Press half of the cookie dough into the prepared baking dish. Pour the cooled chocolate mixture over the dough and spread evenly. Top with small dollops of the remaining cookie dough; don't worry if the dollops don't completely cover the chocolate mixture. Bake for 20 to 25 minutes, until the top is lightly browned. Let cool completely before cutting into bars.
The bars can be stored in an airtight container for up to 4 days.
The cranberries can be omitted from the cookie dough if desired.