Pre-heat oven to 180 C and grease and line a 20cm x 20cm brownie tin.
Whisk the eggs and sugar together in the bowl of a stand mixer until thickened. Add the melted butter, plain flour and cocoa powder and mix to combine.
Spoon 1/3 of the brownie batter into the greased and lined tin and spread out with the back of a spoon - you’re trying to form a sort of reservoir for the condensed milk, so try to keep the centre lower than the edges. Sprinkle with 1/2 of the chocolate chips and bake for 5 minutes, until the top of the brownie is just starting to form a crust.
Remove from the oven and spoon 1/3 of the remaining brownie batter around the sides of the tin - again, trying to form a reservoir. Fill this reservoir with the condensed milk and the gently spoon the rest of your brownie batter on top of the condensed milk. Smooth with the back of a spoon and sprinkle over the remaining chocolate chips.
Bake for a further 25 - 30 minutes until just set and then leave to cool in the tin.