Preheat the oven to 180C (fan). Line up the square tin (I'm using 8 inch tin) with baking parchment, leaving some excess to overhang as it will help when removing brownies from the tin.
In a large bowl, mix together the sugar and coconut oil. Add cashew milk and vinegar, melted and cooled chocolate and vanilla extract. Mix it all well. In a separate bowl mix the flour, cocoa, baking powder, baking soda and salt, then fold them into the bowl with 'wet' ingredients. Stir it all well until just combined, then pour the mixture into prepared baking tin.
Place the tin in the oven and bake for 25-30 minutes, until the inserted toothpick comes out almost clean. Other way to tell if it's ready is when the top has shiny, papery crust and the sides are just beginning to come away from the tin. Set aside to cool completely.
For the peanut butter frosting:
Place all the ingredients in a bowl of an electric mixer and mix until well combined. You may want to add more/ less cashew milk, depending on desired consistency.
Once brownie is completely cool down, spread the peanut butter frosting on top of it, then place in the fridge for about an hour, allowing it to firm up a bit. Slice into square pieces. Enjoy!