Use a very ripe banana for the best flavor and sweetness—deep brown bananas are ideal., Do not overmix the dough to avoid dense, dry cookies; mix just until flour disappears., Underbake slightly for soft, gooey centers; cookies continue baking slightly as they cool., Chocolate chips can be swapped with dark chocolate chunks, milk chocolate, white chocolate, or peanut butter chips., Optional mix-ins include 1/2 cup chopped walnuts, pecans, toasted oats, or shredded coconut., Increase mashed banana to 1/3 cup and add 1-2 tablespoons extra flour for a more banana flavor and to prevent cookies from spreading too thin., Swap vegetable oil with melted butter or coconut oil for richer flavor; browned butter adds a toasty flavor but is not vegan., For gluten-free option, use a cup-for-cup gluten-free baking flour., For vegan option, replace egg yolk with flax egg as described., Store cooled cookies in airtight container for 3-4 days at room temp with a slice of bread to keep moist., Refrigerate cookies up to 5-7 days; warm before serving to restore softness., Freeze baked cookies up to 3 months; thaw at room temp or warm in 300°F oven 5-8 minutes., Freeze dough balls up to 3 months; bake straight from frozen adding 1-2 minutes to baking time., Dough can be refrigerated shaped for 24-48 hours to intensify flavor; let come to room temp before baking if stored longer., To reheat, microwave one cookie for 6-10 seconds or warm several in 300°F oven for 5-8 minutes.