1. In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and confectioner sugar together on low speed until smooth.
2. Add kosher salt and coffee powder (optional) and mix well.
3. Sift together ingredients A (plain flour, rice flour and almond powder) in a small bowl, then stir it all into the creamed butter mixture from #1-2 until it’s completely incorporated, using a silicone spatula.
4. Pat the dough into a rectangle about 5cm thick, wrap in plastic, and chill for about 1 hour.
5. Transfer the dough to a lightly floured surface and roll the dough out. To prevent it from sticking everywhere, I keep it between 2 sheets of baking paper (or saran wrap) and roll until it's 1~1.5cm thick. Cut circles in the dough with your tart or cake rings. Leave the cookies in the rings and place on your baking tray lined with baking paper.
6. Pre-heat the oven to 170℃ degree. Beat the egg in a small bowl and brush the top of each cookie with the egg wash. Then with a fork (or edge of a knife), make a "cross hatch" pattern on the top of each cookie.
7. Bake for about 20 minutes. Leave it to cool on a wire rack.