Preheat the oven to 200°C/fan180°C/gas 6. Sift the flour, baking powder and 1/4 teaspoon salt into a large bowl. Stir in the ham, sage and Cheddar. Make a well in the centre, then add the butter, egg, mustard and milk. Mix together briefly, until just combined.
Put teaspoonfuls of the mixture into petits fours cases, so that each is about three-quarters full. Place on 2 large baking sheets and bake for 8-10 minutes, until risen and golden. Transfer to wire racks to cool a little.
To serve, peel off the petits fours cases and line the muffins up on serving plates. Serve warm or at room temperature. Put your favourite chutney into a serving bowl for guests to dunk the muffins inches.
Tip: Make the muffins, then cool and freeze in food bags for up to 1 month. Defrost in the fridge overnight, then serve at room temperature.