Preheat the oven to 350°F (180°C) and line a 6-inch (15 cm) baking pan with parchment paper or grease it with oil.
In a large bowl, whisk together Greek yogurt, egg, maple syrup, lemon juice and vanilla extract until smooth.
Sift in the cornstarch and protein powder, then fold gently until fully combined; there should be no lumps left in the batter.
Pour the batter into the prepared pan and bake for about 30 minutes, or until the top is golden and a toothpick inserted into the center comes out clean.
Let cool for at least 15 minutes before frosting and serving.
Add all ingredients to a bowl and mix until combined, adjusting sweetness to taste.
Using a spatula or spoon, gently spread the frosting on top of the lemon loaf, once it has cooled.
