Preheat oven to 350°F (175°C) and line or lightly grease an 8×8-inch or 9×9-inch baking dish.
In a medium bowl, whisk together oat flour, protein powder, baking powder, and salt until evenly combined.
In a large bowl, whisk Greek yogurt, eggs, honey or maple syrup, and vanilla extract until smooth and creamy.
Gently fold in the drained crushed pineapple and shredded coconut until evenly distributed.
Gradually add the dry ingredients to the wet mixture, folding gently until just combined. Avoid overmixing.
Transfer the batter to the prepared baking dish and smooth the top evenly.
Bake for 30–35 minutes, or until the center is set and a toothpick inserted comes out with a few moist crumbs.
Allow the cake to cool in the pan for at least 20 minutes before slicing and serving.