Line your baking dish with parchment paper for easy removal later. Melt the butter in a medium saucepan over low heat, stirring gently until it’s completely melted and fragrant.
Whisk in the unsweetened cocoa powder until the mixture is smooth, glossy, and dark in color, with a gentle crackle as the cocoa hits the butter.
Pour in the sweetened condensed milk and stir quickly, watching the mixture thicken and become sticky with a sweet, milky aroma filling the air. Keep stirring until fully combined and glossy.
Add the vanilla extract and a pinch of salt, stirring until evenly incorporated. The mixture should deepen in flavor and smell rich and inviting.
Fold in the chocolate chips and oats (if using), mixing until they’re evenly distributed throughout the thick, fudgy mixture.
Transfer the mixture into your prepared dish, using the spatula to press it down firmly and evenly, creating a smooth top surface. The mixture should feel thick and pliable.
Place the dish in the fridge and chill for at least 2 hours, allowing the mixture to set and firm up with a chewy, fudgy texture.
Once chilled, lift out the block using the parchment paper and place on a cutting board. Use a sharp knife warmed slightly under hot water to cut into neat squares or rectangles.
Serve immediately for the best chewy, chocolatey experience, or store in an airtight container in the fridge for up to a week. For longer storage, wrap tightly and freeze for up to 3 months.
