Preheat the oven to 170°C (340°F) and line your baking tin with parchment paper.
Melt the chocolate gently in a microwave-safe bowl or over a double boiler until smooth. Set aside to cool slightly.
In a large mixing bowl, whisk together the yogurt, coffee, coconut sugar, and melted chocolate until smooth and well combined.
Add the oat flour, cocoa powder, baking soda, and salt. Mix until fully incorporated and a thick batter forms.
Pour the batter into the prepared baking tin and smooth the top.
Bake for 25–30 minutes, or until the center is set but still slightly fudgy. Allow to cool completely in the tin.
Meanwhile, prepare the topping by whisking together the cream cheese, yogurt, liquid sweetener, and instant coffee until smooth and creamy. Adjust sweetness or coffee strength to taste.
Spread the coffee topping evenly over the cooled brownie base. Dust with cocoa powder to finish.
Refrigerate for at least 1 hour before slicing into squares. Store leftovers in an airtight container in the fridge for up to 5 days.