Unsalted Butter (Best Option)
1:1 (1 cup butter per 1 cup coconut oil)Butter works well because it has a similar fat content to coconut oil and melts during baking to create a tender crumb. The milk solids in butter also contribute to browning and flavor development through Maillard reactions.
For best results, use melted butter to match the liquid state of coconut oil and ensure even mixing. Be mindful that butter contains water, so the batter might be slightly wetter; reduce other liquids slightly if needed.
Compared to coconut oil, butter will add a richer, more buttery flavor and a slightly softer texture, which many find desirable in banana bread.