Preheat the oven to 180°C (350°F). Line a 20cm (8-inch) square baking pan with baking paper.
To make the brownies: Add all the dry ingredients to a medium-size bowl and mix until there are no lumps. Add all the wet ingredients to the bowl and mix until combined and smooth. Pour the brownie batter into your baking pan and smooth the top.
Bake the brownies in the oven for 15-20 minutes (shorter if you like them fudgy, longer if you would like them more crispy). The brownies are ready when the surface is dry. They will set more over time.
Allow the brownies to cool in the square baking pan. Cover the pan and place them in the fridge overnight to set.
To make the frosting: Add the vegan butter to a large mixing bowl or stand mixer. Use electric beaters or a stand mixer to whisk the butter on high for 5-10 minutes or until it has become pale yellow and is light and fluffy. Add all the powdered sugar and vanilla extract to the bowl. Whisk the frosting on low for 1 minute or until combined. Increase the speed of the beaters or stand mixer to high and whisk for 3 minutes or when the frosting is light and fluffy.
Divide the frosting into 3 bowls. Add the beetroot powder to one bowl and the blue spirulina to another bowl. Mix the frosting with the powders until combined. Decorate the cooled brownies as desired. Slice to serve.
Depending on your climate, the brownies can be stored in an airtight at room temperature up to 1 day. Alternatively, store them in the fridge for 1 week or in the freezer for up to 1 month.