Position a rack in the center of the oven and heat to 375°F (190°C). Lightly grease an 8-inch square baking pan or line with parchment.
Spread the chopped walnuts on a small baking sheet and toast in the oven for 6–8 minutes until fragrant. Let cool briefly, then reserve about a tablespoon for garnish.
In a medium bowl whisk together the oats, baking powder, cinnamon, cloves, ginger, nutmeg and salt until evenly combined.
In another bowl mash the bananas with a fork until mostly smooth. Whisk in the almond milk, maple syrup, molasses, melted coconut oil and vanilla until blended.
Pour the wet mixture into the dry ingredients and fold gently to combine. Stir in the toasted walnuts, leaving a handful to scatter on top. The batter will be thick — let it rest for 10 minutes so the oats absorb the liquid.
Scrape the mixture into the prepared pan and smooth the top with a spatula. Sprinkle the reserved walnuts over the surface.
Bake 28–32 minutes, rotating the pan halfway through, until the edges are lightly browned and the center is set (a toothpick should come out mostly clean). Remove from the oven and allow to cool in the pan for about 10 minutes.
While the oatmeal cools, make the drizzle: place the coconut butter, almond milk and maple syrup in a small heatproof bowl and warm over a pan of simmering water or microwave in 20–30 second bursts, stirring until pourable and smooth.
Drizzle the warmed coconut butter mixture over the oatmeal, cut into 6 squares and serve warm. Leftovers can be reheated briefly in the microwave.