Chocolate Oat Dulce De Leche Bars

flour 1 cup oats 3/4 cup brown sugar 1/4 cup cocoa powder 1/2 tsp espresso powder 1/2 tsp baking soda 1/4 tsp salt 1/2 cup unsalted butter 2 ounces unsweetened chocolate 1 13.4-ounce can dulce de leche 2 Tbsp milk 1 cup semisweet chocolate chips