Preheat the oven to 160°C for fan forced or 180°C for conventional. Grease and line two square 20cm cake tins.
Add ½ cup of boiling water to a jug and combine ½ tsp of coffee; mix well, and set aside to cool.
Sift flour, cocoa powder, baking powder and bi-carb soda into a mixing bowl. Then add the white sugar and salt and combine.
Add the buttermilk, egg, vanilla extract and vegetable oil. Then mix on a low speed until just combined. Add the coffee and mix on low at first, then on high until well combined.
Pour the cake mixture into the 2 cake tins as evenly as possible and bake for 15 minutes or until cooked.
Once cooked, allow the cake slice to cool for about 10 minutes before transferring them to a cooling rack to cool completely.