Preheat the oven to 350°F (175°C) and prepare your skillet by greasing it lightly with butter or cooking spray.

Melt sweetened condensed milk and butter together in a saucepan over low heat until smooth and creamy.

Stir in the chopped bittersweet chocolate into the melted mixture until fully melted.

Remove from heat and combine the vegetable oil into the chocolate mix. Whisk in the eggs and egg yolk until well blended.

Add the vanilla extract and mix thoroughly.

In a separate bowl, whisk together the dry ingredients: all-purpose flour, unsweetened cocoa powder, baking powder, kosher salt, light brown sugar, and granulated sugar.

Gradually combine the dry ingredients with the wet mixture, gently folding until just combined.

Pour the brownie batter into the prepared skillet and smooth the top with a spatula.

Bake in the preheated oven for about 25-30 minutes, or until set but still a bit soft in the center.

Let cool for about 10 minutes before cutting into squares.