Preheat the oven to 325º F. Lightly grease your loaf pan, unless it's an uncoated stoneware pan.
Combine the butter, brown sugar, and vanilla extract in a medium bowl. Mix by hand until smooth, about 2 minutes. Or use a stand or hand mixer on low speed for 1-2 minutes until the mixture is smooth.
Stir in mashed bananas and eggs. Combine until smooth, though there might be small lumps of banana. Add Bisquick baking mix, 1/2 cup at a time, stirring in between. Then add nuts, if using.
Pour the banana bread mixture into the prepared loaf pan. Smooth the top before baking.
Bake at 325º F for 45-55 minutes or until a toothpick inserted into the middle of the loaf comes out clean. If the banana loaf gets too brown, cover it with foil during baking.
Allow bread loaf to cool for 10 minutes in the pan before removing and slicing to serve.
Notes
Get our recipe to make your own Bisquick in our DIY Homemade Baking Mix post. Store leftover bread in an air-tight container at room temperature for up to 5 days. Freeze for longer storage (up to 3 months).