- Line the cookie sheets with parchment paper.
- In the food processor, grind the almonds and icing sugar until they form a fine powder, 2 to 3 minutes. Sift and set aside.
- Using an electric mixer, beat the egg whites with the food colouring until soft peaks form. Gradually add the sugar, beating constantly until the peaks are stiff. Using a spatula, fold in the almond–sugar mixture and stir until the mixture deflates and softens slightly.
- Using a pastry bag fitted with a 1.5-cm (1/2-inch) plain tip, pipe 2.5-cm (1-inch) macaroons onto the cookie sheets, spaced about 5 cm (2 inches) apart. The macaroons should flatten slightly; if they do not, the batter has not been stirred enough. Let dry at room temperature until the macarons develop a thin crust, about 90 minutes.
- With the rack in the middle position, preheat the oven to 180°C (350°F).
- Bake, one sheet at a time, with the oven door propped open with a wooden spoon, 12 to 15 minutes. The macarons should not brown.
- Let cool completely. Spread a thin layer of marmelade on the underside of a macaron and cover with another macaron. Repeat with the remaining macaroons and marmalade.
- The macarons will keep in an airtight container for about 1 month without marmalade or 2 weeks with marmalade. They also freeze well.