Prepare and measure all ingredients using measuring cups.
Place the oven rack in the center position and preheat the oven to 350 F. Spray an 8-inch square baking pan with nonstick cooking spray. Line with parchment paper, leaving some hanging out opposite sides like handles. Spray again, then set aside.
In a small saucepan, bring 1/2 cup Guinness beer to a boil. Boil for 5 minutes, until reduced by half (you should have 1/3 cup reduced syrup). Set aside to cool.
In a small mixing bowl combine together 3/4 cup oat flour with 1/4 cup cacao powder. Add pinch of salt, mix and set aside.
In the microwave or a double boiler, melt 1 1/2 chopped dark chocolate and 3/4 cup unsalted butter or coconut oil until completely smooth. Set aside to cool slightly.
In a large mixing bowl, whisk 3/4 cup coconut sugar with 3 eggs, and 1/2 teaspoon vanilla extract.
Add melted dark chocolate mixture and 1/3 cup Guinness syrup. Whisk to combine.
Stir in flour and cacao mixture. Keep stirring until mixture is smooth and glossy. Use the whisk.
Pour the brownie mixture into prepared pan. Bake for 30-35 minutes, until a toothpick inserted into the center comes out clean. Be careful not to overbake!
Carefully take out of the pan and place it on a wire rack to cool completely.
Dust with cacao powder. Slice and serve.
Makes 9 slices.
Tips for storing Keep covered in the refrigerator for up to one week. You can store brownies in an air-tight container or cake carrier. HOW TO FREEZE: Place in a zip-lock bag and freeze up to three months. Thaw on room temperature before using.As a measure, I used US cup (240ml).Disclaimer: The nutritional information provided is approximate and is calculated using online tools. Information can vary depending on various factors, but I have endeavored to be as accurate as possible.