Preheat the oven to 180C/350F/Gas 4. Line an 8x8-inch square baking pan with foil or parchment paper. Set aside.
Whisk together the flour, baking powder, baking soda, and salt. Set aside.
Whisk together the butter and sugars until combined. Add the egg, and vanilla, and mix until combined. Add the dry ingredients, and mix until just combined. Fold in the chocolate chips.
Press half of the dough into the prepared pan.
For the Fudge
Place the chocolate into a microwave-safe bowl, and heat in 30 second intervals until melted and smooth. Add the condensed milk, and whisk until combined.
Spread the mixture into the prepared pan, and smooth out the top. Place the remaining cookie dough on top and sprinkle with extra chocolate chips.
Bake for 25 - 30 minutes or until lightly golden brown on top. Allow to cool completely in the pan on a wire rack. Refrigerate or freeze for at least 2 hours before cutting into bars.
Video
Notes
Bars stay fresh, stored in an airtight container, at room temperature or in the fridge for up to 1 week. They also freeze well for up to 3 months. Thaw overnight in the fridge before serving.