"I know that the essence of shortbread is its buttery plainness, and that adding coffee should be all wrong, but it is – convincingly – so very, very right. Instant espresso powder (a baking essential for me) is needed for these; regular coffee granules would not melt in properly, though you could bash them to a powder, if that's all you can find. I like to press a walnut half into each fat patty as I go; it gives the baked cookies, by then fairly large flattish discs, an elegant look with their nut-defined cen..."