Preheat the oven to 350°F (180°C). Prepare a 9x9 inch square baking dish by spraying with non stick spray and lining with parchment paper so that there is overhang on either side.
Sift gluten free all purpose flour and cocoa powder into a mixing bowl. Add the sugar, baking powder, salt and instant espresso powder and mix together.
Prepare your vegan buttermilk by adding 1 Tablespoon of lemon juice to a measuring jug and then adding soy milk up to the 1 cup (240ml) line. Let it curdle.
Add the vegan buttermilk, melted coconut oil and vanilla extract to the mixing bowl and mix into a thick batter.
Then add the vegan chocolate chips and chunks and mix in.
Transfer the batter to your prepared baking dish and smooth down.
Bake for 40 minutes. I found 40 minutes to be perfect, they have plenty of fudginess in the texture but they're not ultra gooey. See notes for how to know they are done.
Let them cool for 15 minutes and then lift the whole tray of brownies out using the parchment paper overhang. Let the brownies cool completely before cutting. If they're still warm when you cut them, they're harder to cut and more breakable.
To cut the brownies, heat up a knife by dipping it in some very hot water (from the kettle) and then drying it off with a towel and cutting. After each cut, dip the knife in the hot water and dry it off again, and repeat. This allows you to cut perfect squares and also melts the chocolate along the edges as you cut the squares.