Step 1

Preheat: Preheat oven to 350°F (177°C).

Step 2

"Flax egg": In a small bowl, whisk together flax and water. Set aside for 10 minutes to thicken.

Step 3

Walnut Streusel: Add ½ cup of the oats to a food processor, pulse into powder. Transfer oats to a medium bowl. Pulse walnuts until finely chopped. Transfer walnuts to the bowl. Add remaining ½ cup (whole) oats, sugar, coconut oil, cinnamon, and salt. Stir until clumps form. Set aside.

Step 4

Dry ingredients: In a large bowl, whisk together spelt flour, cocoa powder, baking powder and baking soda. Set aside.

Step 5

Wet ingredients: In a medium bowl, whisk together banana, sugar, milk, coconut oil, vanilla, and salt. Add "flax egg", whisk until combined.

Step 6

Combine: Add the wet ingredients to the dry ingredients. Gently mix until there are a few streaks of flour left. Gently fold in walnuts and chocolate (be careful not to overmix).

Step 7

Assemble and bake: Line 1 9-inch square pan with parchment (see note 2). Pour the batter into the pan, then sprinkle the walnut streusel on top. Bake until a toothpick comes out clean, 40 to 50 minutes. Let cool, lift from pan, and cut into squares.