Bizcocho de avellanas

The most common cause is overbaking, so start the toothpick test toward the lower end of the 40-45 minute window rather than waiting for the timer to run out. A second cause is overmixing after the flour goes in - the recipe says to fold in the flour, baking powder, and hazelnuts and to mix only until the batter is smooth, because working it longer develops the flour and makes the crumb tough. Also make sure the butter is genuinely soft before creaming, since cold butter won't hold the air that keeps the cake light.

Yes, ground almonds or walnuts work in the same 80 g quantity, since the nuts here act as part of the dry mixture folded in with the flour. If you only have whole hazelnuts, grind them finely first - coarse pieces don't absorb moisture the way ground nuts do and will change the batter's texture. Toasting the ground hazelnuts briefly in a dry pan and letting them cool before folding them in gives a deeper nutty flavor without altering the recipe.

Let the cake cool completely as the recipe directs before wrapping it, otherwise trapped steam will make the surface soggy. Kept in an airtight container at room temperature, it stays moist for several days - the ground hazelnuts and the 50 ml of milk help it hold moisture. Add the powdered sugar only just before serving, since it dissolves into the cake as it sits.

You can grease and flour the loaf pan and weigh out all the dry ingredients - flour, baking powder, ground hazelnuts, sugar, vanilla sugar, and salt - hours ahead. The finished batter, though, should go straight into the preheated oven, because the baking powder starts working as soon as it meets the milk and eggs and loses lift if it sits. The baked cake itself is a good make-ahead: bake it a day early, let it cool fully, and dust it with powdered sugar right before it goes on the table.

Yes - cold eggs added to the creamed butter can make the mixture curdle or split, which costs you some of the air you beat in over those 2-3 minutes. Take the 2 eggs and the 50 ml of milk out along with the butter so everything is at room temperature, and add the eggs one at a time as the recipe says, beating about 30 seconds after each. If the mixture does look separated, a spoonful of the measured flour stirred in will usually bring it back together.