Sift together the flour and salt into a bowl. Add the egg and part of the milk and whisk well to give a smooth batter. Mix in the rest of the milk until smooth and even. The batter will be relatiely thin. Set aside for around 15 - 20 minutes.
Warm a 12in (30cm) wide skillet/frying pan (recommend nonstick or cast iron) over a medium-high heat. Melt around 1 teaspoon of butter and swirl around the pan to give an even thin, layer.
Reduce the heat slightly and add somewhere between ⅓ and ½ cup (around 100ml) of the pancake batter to the pan. Tilt the pan slightly after you have added the batter and swirl a little so the batter spreads out to the edges.
Let the pancake cook a couple minutes until the top is dry - if you lift up an edge it should be gently brown. Loosen the pancake with a spatula to ensure it is not sticking, then flip the pancake over and cook a minute or two more on the other side. Remove the pancake from the pan once the other side gets a few brown spots and keep warm while you cook the rest.
Topping-specific instructions
If adding speck/ham and cheese to the pancake - slice or tear up around 1 - 2 slices of speck, prosciutto or ham into smallish pieces. Cut around 5 thin slices of gouda. To cook, after adding the pancake batter, add pieces of speck/ham into the batter while it is still wet so they hold in place. Then, after cooking on the second side, flip the pancake back over so the speck is on top then add the slices of cheese. Cover with a lid to help the cheese melt and cook a minute more on low heat until cheese is melted. (If using only one or other then skip the part you are not adding)
If adding apple into the pancake - cut around 5 very thin slices of apple and remove core and skin. Add slightly more butter than usual and cook the apple slices a minute or two either side to soften before then adding the batter over the top. Then, cook as usual, though you may need an extra spatula to help turn as they will be more delicate.
For other toppings (eg syrup, slices of strawberry, caramelized apples, cinnamon sugar) - add the toppings when cooking the second side or once served onto plate.
Notes
If the batter seems too thick for some reason, you can easily thin it with a little extra milk.To keep the pancakes warm, I usually use a dish cloth folded over so it covers over and under the pancakes, and add the pancakes on top of the others as they cook. If you need to re-heat these, you can either warm them gently in the skillet or give a short heat on the microwave. If in the skillet, the edges may crisp a little. If in the microwave, make sure you cover with kitchen roll. I originally shared a version of these with part water, to make a slightly less rich batter, but based on reader feedback and going back to the recipe I have amended to just milk. If you prefer a slightly less creamy/rich batter, then use around ¼ water or else use a lower fat milk. I would not, however, suggest using fat free/skimmed milk.