Line 2-3 large baking trays with nonstick baking paper. Using a small cookie cutter and a pencil, mark out 50 circles roughly 3-4cm (1½in) apart. Combine the icing sugar and ground almonds in a bowl, set aside. Put the egg whites, egg white powder (if using), and a pinch of salt in a clean bowl and whisk until soft peaks form. Gradually add the caster sugar and whisk until the mixture resembles a thick, glossy meringue that holds stiff peaks. Use a little of the mixture to secure each corner of the baking paper to the tray, then gently fold in the vanilla extract and food colouring until it reaches the desired pastel colour. Add the icing sugar and ground almond mixture to the bowl, using a spatula to fold everything together until smooth for precisely 60 seconds. Use a spatula rather than a wooden spoon as this will deflate the mix. Put a 1cm (½cm) plain nozzle into a piping bag. Spoon the meringue mixture into the bag and pipe out 50 discs. Set aside to dry for 1 hour. They should form a non-sticky skin. Preheat the oven to Gas Mark 3, 170°C, fan 150°C. Bake the macarons for 10-12 minutes, until just set and crisp*. Remove from the oven and gently slide the baking paper, along with the macarons off the tray. Sprinkle the empty, hot tray with cold water, then return the baking paper and macarons to the tray and set aside to cool. The effect of the cold water will make removing the macarons easier when they're ready. Once cold, transfer the macarons to a wire rack. To assemble the macarons, whisk together the buttercream ingredients until combined and creamy. Spread a little buttercream onto the base of one macaron half, then sandwich together with another half. Repeat with the remaining macarons and leave to set. *Babies, toddlers, pregnant women, the elderly or people who are unwell should avoid lightly cooked or raw eggs.