Line your square baking dish with parchment paper, leaving some overhang for easy removal later.
In a microwave-safe bowl, melt the butter and honey together in short bursts of 30 seconds, stirring between each until the mixture is smooth and warm, about 45°C (113°F).
Add the sifted cocoa powder to the melted mixture and stir until fully combined, releasing a rich chocolate aroma.
Pour in the oats, and if using, add chopped nuts or dried fruit. Mix everything together until the oats are evenly coated and the mixture is thick and sticky.
Transfer the mixture into the prepared dish. Use a spatula to press it down firmly and evenly, creating a compact, uniform layer.
Scatter the chocolate chips or chunks over the top, pressing them gently into the surface so they melt slightly and create a glossy finish.
Place the dish in the fridge and chill for at least 2 hours, or until the bars are firm and set.
Once chilled, lift the set mixture out using the parchment overhang and place on a cutting board. Use a sharp knife to cut into bars or squares.
Serve immediately for a fudgy, chewy treat, or store in an airtight container in the fridge for up to a week.
