Preheat your oven to 180C/360F. Grease a 33 x 22cm (13 x 9 inch) cake tin with butter, and line with baking paper.
Mix together the 1 tsp instant coffee granules with 3/4 cup boiling water and set aside.
In a large bowl, sift together 1+3/4 cups gluten-free plain/all purpose flour, 3/4 cup cocoa powder, 2 tsp baking powder, 1.5 tsp baking soda.
Add the 1.5 cups castor/white sugar, 3 eggs, 1 cup milk, 1/2 cup vegetable oil, and 1 tbsp vanilla extract. Mix to combine until the batter is lump free.
Add the coffee mixture to the cake batter and mix to combine. You will now have a glossy, smooth chocolate batter that is quite thin.
Pour your cake batter into the prepared cake tin and bake for 40 minutes, or until a skewer inserted into the middle of the cake comes out clean. I also do a quick light press on the top, and if it springs back, it's usually ready!
Allow the cake to cool for about 5 minutes, then turn it upside down onto a wire rack to cool. By turning it upside down, you can flatten out any 'doming' that may have occurred.
Let the cake cool completely before you begin cutting, and/or decorating it.