Preheat the oven to 180°C, and grease your cake tin with a little butter. If, like me, you’re using a silicone cake tin, you can skip the greasing, as the cake will unmould without a hitch.
Put the flour into a bowl, and stir in the sugar and salt. You can use a freestanding mixer if preferred; I’ve always found this soothing to do by hand.
Add the butter and mix until the dough resembles coarse breadcrumbs. Add about 3/4 of the egg, and knead into a smooth, slightly sticky dough.
Scoop this dough into the tin, and press firmly down until evenly distributed. Add the remainder of the egg, and smooth the top with the back of a spoon.
Place in the oven to bake for 30-40 minutes, until the cake is golden brown on top. Mine is always ready precisely at 30 minutes, so be sure to start checking after 30 minutes.
Transfer the tin to a wire rack and leave to cool for 10 minutes before carefully unmoulding, using your fingers to help carefully prise the cake away from the edges if it’s dragging its feet. I recommend cutting the cake into small squares; it will feel slightly brittle at this point, but it will firm up later.