Whisk milk, eggs and vegetable oil together in a bowl. Sift in plain flour and buckwheat flour gradually, stirring constantly, until batter is quite runny.
Melt butter in a 20cm frying pan or crepe pan over medium heat. Pour 1/4 cup batter into the pan. Rotate the pan until batter coats the bottom in a thin layer. Cook until the top of the crepe is no longer wet and the bottom has turned light brown, 1 to 2 minutes. Run a spatula around the edge of the pan to loosen crepe; flip crepe and cook until the other side has turned light brown, about 1 minute more. Transfer to a cooling rack. Repeat with the remaining batter, whisking gently before using.
Use a milk substitute in place of the milk if desired.