Add the Greek yogurt, protein powder, eggs, honey, vanilla, and espresso powder to the bowl with the bananas. Whisk until everything looks smooth and evenly combined with no big streaks of protein powder.

In a separate bowl, whisk together the oat flour, baking powder, baking soda, cinnamon, nutmeg, and salt, breaking up any little clumps.

Pour the dry ingredients into the wet. Gently fold everything together until just combined; stop as soon as you don’t see dry flour pockets. A few small lumps are okay—overmixing will give you a tougher, rubbery cake.

In a small bowl, stir together the oat flour, crushed walnuts, cinnamon, espresso powder, and sweetener.

Drizzle in the melted coconut oil and use a fork (or your fingers) to toss everything together until you get coarse, clumpy crumbs that look like small pebbles. If it looks sandy, keep squeezing it together.

Preheat your oven to 350°F (175°C). Line an 11×7‑inch baking dish with parchment and lightly spray the parchment and sides with oil so the cake releases cleanly.

Pour the batter into your prepared pan and spread it into an even layer, pushing it all the way into the corners.

Sprinkle the crumb mixture evenly over the top, breaking up any huge clumps but keeping some nice chunky pieces for texture.

Bake for 35–45 minutes, or until the top is golden and a toothpick inserted in the center comes out clean or with just a few moist crumbs attached. If the top is browning too quickly, you can tent the pan loosely with foil near the end.

Let the coffee cake cool in the pan for at least 30 minutes so it can set up.

Slice into squares and serve with coffee.