Preheat your oven to 350°F (175°C) so it’s fully hot by the time the batter is ready. Line a loaf pan or square baking dish with parchment paper, leaving a little overhang on the sides, or lightly grease it to prevent sticking and make removal easy later.

If you’re using whole rolled oats, place them in a blender or food processor and pulse until they turn into a fine, flour-like texture. The oats should look sandy and even, not pasty, as this helps the cake bake evenly and stay light.

In a large bowl, add the ground oats, protein powder, baking powder, cinnamon, and salt. Whisk well so the baking powder and protein powder are evenly distributed, which helps the cake rise properly and prevents dense spots.

In a separate bowl, whisk together the eggs, yogurt, oil, sweetener, and vanilla until the mixture looks smooth and slightly glossy. Make sure the yogurt is fully blended so there are no thick lumps in the batter.

Add the grated apple and grated carrot to the wet mixture and gently fold them in with a spatula. Stir just enough to evenly distribute the fruit and vegetables without breaking them down too much, as they provide moisture and texture.

Add the dry ingredients to the wet mixture and fold gently until just combined. Stop mixing as soon as you no longer see dry pockets of flour; overmixing at this stage can make the cake dense instead of soft and tender.

Pour the batter into the prepared pan and use a spatula to smooth the top evenly. Lightly tap the pan on the counter once or twice to release any large air bubbles.

Place the pan in the center of the oven and bake until the cake is set in the middle and lightly golden on top. A toothpick inserted into the center should come out mostly clean with a few moist crumbs.

Remove the cake from the oven and let it cool completely in the pan. Cooling allows the cake to firm up and develop its final texture, making it much easier to slice cleanly without crumbling.