Preheat your oven to 180°C (350°F) and line an 8-inch square baking tin with parchment paper, leaving some overhang for easy removal.
Slice 5 strawberries in half and set aside (for the top of the brownies), then mash the rest of the strawberries with a fork and set aside.
In a mixing bowl, whisk together the yogurt, coconut sugar, egg, egg yolk, milk, and vanilla extract until fully combined.
Add the oat flour and cocoa powder to the wet ingredients and gently fold until almost combined.
Fold in the mashed strawberries and optional dark chocolate chips or chunks.
Pour the brownie batter into the prepared baking tin and smooth the top.
Arrange your sliced strawberries on top of the batter, spacing them evenly. Optionally, add more dark chocolate chips on top.
Bake for 40-45 minutes, depending on how fudgy you prefer your brownies. For extra fudginess, aim for 40 minutes.
Allow the brownies to cool completely in the tin before slicing with a clean, sharp knife.