Preheat oven to 350°F (175°C). Line an 8x8 inch baking dish with parchment paper.
Grind oats into a fine flour using a blender or food processor.
In a large bowl, whisk eggs and monkfruit until smooth. Add vanilla and yogurt, and mix again.
Sift in oat flour, cocoa powder, baking powder, baking soda, and a pinch of salt.
Stir gently to combine. Batter will be thick—add milk to loosen it.
Pour batter into the prepared baking dish.
Bake for 40–45 minutes or until a toothpick inserted in the center comes out clean.
Let the cake cool slightly before slicing.
Optional: Melt chocolate with milk and drizzle over cake before serving.