First, preheat the oven to 350°F (175°C) and cover a baking pan (8x8 inch) with baking paper and grease with unsalted butter.
Combine half of the chocolate chips with butter and unsweetened chocolate in a large bowl (heat-proof). Melt the chocolate au bain-marie or in the microwave by heating it in 30-second bursts and stirring between each burst, until the mixture is well melted and smooth. Let the mixture cool.
Add the eggs and sugar to the melted chocolate and mix on medium-high speed until it is for 3 minutes. Then add the vanilla extract. Fold the flour and salt into the mixture. Pour half of the mixture into the prepared baking pan, and bake for 10 minutes.
Pour the sweetened condensed milk over the top and the remaining half of the chocolate chips. Pour the remaining mixture over. Put it back in the oven and bake for 35 minutes, but do not overbake.
When the baking process is finished, let cool to room temperature before placing them in the fridge to chill.
Serve the brownies at room temperature or warmed in the microwave.