Butter is mostly fat, but it also has milk solids and water. When you heat it too high, those milk solids start to burn. That creates a smoky, dark mess – and a flavor that tastes like regret.
The smoke point for butter is around 350°F (175°C). That’s lower than you think. Your oven often runs hotter than that. So, butter in the oven is a tightrope walk.