Preheat the oven to 350 degrees F. Lightly butter or add parchment paper to one 9-by-5-inch loaf pan or three 6-by-3-inch mini loaf pans.
Whisk the flour, oats, baking powder, salt, baking soda, and cardamom in a large bowl. Add the chopped dates and walnuts.
Whisk the wet ingredients: add sugar, eggs, oil, yogurt, and vanilla in a medium bowl or in your mixer. Stir in the mashed banana. Fold the banana mixture into the flour mixture until just combined.
Spread the batter in the prepared pan(s). Bake until a toothpick inserted into the center comes out clean, about 55 minutes for a standard loaf or 35 to 40 minutes for mini loaves. Cool 30 minutes in the pan on a rack, then turn out onto the rack to cool completely.