Preheat oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper or foil, lightly greased.

In a large bowl, whisk together melted butter, egg, brown sugar, granulated sugar, and vanilla extract until well combined.

Add flour, cocoa powder, baking soda, and salt. Stir until just combined. The dough will be thick.

Reserve about 2/3 cup of the cookie dough. Press the remaining dough evenly into the prepared baking pan.

Sprinkle the chopped chocolate candy bars evenly over the dough in the pan. Crumble the reserved cookie dough over the top of the chocolate.

Drizzle the sweetened condensed milk evenly over the crumble topping.

Bake for 27-30 minutes, or until the edges are set and the center is mostly cooked through. The condensed milk should be slightly thickened.

Let the bars cool completely in the pan before cutting into squares. This is crucial for them to set properly.