The shortbread I made was soft at that time, but it became hard the next day.

Let me tell you, let me analyze several reasons: 1. It's not fully baked, and it sticks to your teeth when you eat it. This kind of cake is soft when it is hot, but it becomes dough when it is cold. It is hard to be hard or not. This has to be determined from several aspects. In appearance, are both sides golden? Not a small part of the cake, but baking and coloring. When cut open, how many layers are there in the crust, and how many layers have no discoloration at all? At most, only two layers are allowed without coloring. 2. The ratio of cake crust to pastry is wrong. 3. Is the filling incomplete? Did the sugar in the stuffing stick to the pan? 3. The stuffing is too dry. 4. The formula is wrong. Then there was nothing to do but make a cake like a stone. The official account of WeChat, "Shorty Master Wang Pai", has a detailed explanation.