(Note that you can buy canned Dulce de Leche on the Hispanic foods aisle in many grocery stores and skip this step! You can also make it in a crock pot or stove--look up these other methods online. Microwave is faster, but has some danger involved.)
Combine sweetened condensed milk and corn syrup in a LARGE microwave safe bowl. You need the bowl large because the stuff may start to boil like an erupting volcano.
Microwave on 50% power, stopping and stirring every 2 minutes. Start watching VERY CAREFULLY about the 5 minute point. It can start to boil over very quickly. Start doing 1 minute increments, stirring well in between. Somewhere from 10 to 15 minutes, it will thicken and look caramel-colored rather than white, at which point it is ready. It might look curdled but that goes away when you stir. Add in the butter and stir until it's melted. Set aside.
To Make the Brownies
Preheat oven at 325-degrees. Line a 9x13 pan with foil and butter it or apply nonstick spray.
In a large bowl, cream the butter and sugars until fluffy. Beat in eggs and vanilla. Combine flour, espresso powder (if using), cocoa, and salt. Add dry ingredients into creamed mixture; mix until well blended.
Spread about 1/2 of mixture in the prepared pan. This will just cover the bottom. Drop spoonfuls of Dulce de Leche evenly over this layer and smooth out with an uneven spatula. Sprinkle chocolate chips all over. Cover with remaining brownie batter using clean uneven spatula.
Bake 35 to 40 minutes, until the middle passes the toothpick test. Cool and cut into squares.