Line a 9x9-inch square baking pan with parchment paper making sure to leave some overhang on the sides; this helps make it easier to pull the bars out and cut into squares. You could also spray the pan with cooking spray if wanted.
In a mixing bowl using an electric hand mixer, or use a stand mixer with the paddle attachment, beat the butter, powdered sugar, and light brown sugar until light and fluffy; about 2 minutes.
1/2 cup (1 stick) salted butter, 1/2 cup powdered sugar, 1/4 cup brown sugar
Add in the vanilla extract and beat just until combined.
1 teaspoon vanilla extract
Add half of the flour and half of the sweetened condensed milk and beat on low speed just until combined. Add the remaining half of flour and remaining sweetened condensed milk and beat on low speed just until combined.
2 cups all-purpose flour, 1 can (14 oz) sweetened condensed milk
Stir in the mini chocolate chips.
1½ cups mini chocolate chips
Dump the cookie dough into the prepared pan and firmly press it down to form an even layer inside the pan.*If the dough is too sticky, spray your fingers with cooking spray and then sprinkle a little flour on them. You could also try using a plastic spoon to press the cookie dough into the pan, as that usually helps prevent the stickiness. Cover the pan with a lid or plastic wrap, and refrigerate for at least 4 hours, or up to overnight.
In a microwave-safe bowl, add the milk chocolate chips and oil, heat in the microwave in 30-second increments, stirring after each one, until it's completely melted and smooth.
1 cup milk chocolate chips, 1 teaspoon oil
Pour the chocolate over top the chilled cookie dough bars and smooth it out into an even layer. Immediately sprinkle with the mini chocolate chips.
1/2 cup mini chocolate chips
Refrigerate the bars for 1 hour or until firm. Cut into squares and enjoy! Keep the cookie dough bars stored in the fridge and eat chilled. For a softer cookie dough bar, allow the bars to sit out at room temperature for a few minutes to soften.