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 (⅓ 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.