Pre-heat your cast-iron pan on medium-high heat for at least 5 minutes, then reduce to medium-low.
In a large bowl, whisk together the egg, sugar, salt, and baking powder.
Gradually add the the milk and flour in alternating turns, whisking gently until a lumpy batter forms - do not overmix the batter. Finally, whisk in the 30g of melted butter.
Stick the cold butter to the tines of a fork and use it to butter you pan - it should start foaming and sizzling. Pour about 100 ml of batter for each pancake, letting it spread naturally.
Cook for about 1 ½-2 minutes, or until golden brown with crisp edges, then flip and cook for another 1 minute or so on the other side.
Repeat with the remaining batter, adding more butter to the pan as needed. Serve warm with extra butter and maple syrup.
