Whisk the flour, sugar and salt. Rub in the cold butter with your fingertips (or pulse in a food processor) until it looks like damp sand.

Add the egg yolk, vanilla and 1 tablespoon milk and mix until the dough comes together, adding more milk a teaspoon at a time only if it's dry. Flatten into a disc and chill for 20 minutes.

Heat the oven to 350°F (175°C). Roll the dough to 1/8 inch (3 mm) thick on a lightly floured surface and cut 1-inch (2.5 cm) rounds. Re-roll the scraps.

Place the rounds on parchment-lined baking sheets about 1/2 inch apart and chill for 10 minutes so they stay flat.

Bake for 8 to 10 minutes, until just golden at the edges. Cool completely on the trays.

Beat the meringue powder with 3 tablespoons water until foamy. Add the powdered sugar and beat on medium-high for 4 to 5 minutes, until thick, glossy and holding stiff peaks.

Divide the icing into bowls and tint each with a little gel coloring. Spoon into piping bags fitted with small open star tips (like Wilton #21).

Hold the bag straight up just above a biscuit, squeeze a star that covers most of the top, stop squeezing, then lift sharply to make a peak.

Dry in the oven at its lowest setting, 200°F (95°C) or below, for 45 to 60 minutes, or air-dry for 6 to 8 hours, until the icing is completely hard. Store airtight.