Whisk buckwheat flour, salt, egg and half the milk in a medium sized mixing bowl until it forms a paste. Then pour in the rest of the milk and the maple syrup and whisk well to form a smooth batter.
Heat all the butter in a 9 inch bottomed cast iron skillet pan on a medium-high heat until the butter has melted.
Swirl the pan around so the butter completely covers the pan then pour out the excess butter into a cup and set aside.
Pour 80ml (1/3 cup) of the galette batter into pan, tipping the pan so that the batter thinly covers the bottom of it.
Cook for about 3-4 minutes until the bottom of the galette is getting golden and crisp, then sprinkle on 30g of the gruyere all over the galette plus a few slices of the torn ham.
Then fold the galette in half, and then half again. Remove from the pan and set aside to rest on a warm plate (or in a very low oven) whilst you prepare the rest of the galettes.
Wipe the pan with some kitchen towel to remove any melted cheese or sticky bits of galette.
Pour in a teaspoon or so of the pre-melted butter, swirl the pan to cover the bottom then add another 80ml of the batter to produce another galette and follow the rest of the instructions as before.
Make as many galettes as the batter allows.
Serve the galettes piping hot.