Line a baking sheet with baking paper.
Measure out your almond meal and icing sugar. Blend these until fine, alternatively you could pass these through a sieve a few extra times.
Sift the almond meal and icing sugar twice into a bowl, throw out any bits that don't pass through and set aside.
Place your egg white into a clean metal or glass bowl.
Whip your egg whites with a whisk attachment of an electric stand or hand mixer until foamy.
When the mixture is foamy start to slowly add in your castor sugar.
Continue to whip until all the castor sugar is incorporated into the egg whites and it is thick and glossy with firm peaks.
Add in your drops of colouring and whisk until the colour is mixed through.
Sift 1/3 of your almond/sugar mixture into the egg whites. (Yes we're sifting again)
Gently mix the almond/sugar mixture into the egg whites until it is fully incorporated.
Sift in the rest of the almond/sugar mixture and gently fold through.
Continue to fold until incorporated. The mixture will go from a consistancy of being too thick to fall off the spoon to being able to ribbon off the spoon. You don't want to get this mixture too runny, so if in doubt, stop mixing earlier than you think.
Fill your mixture into a piping or ziplock bag with a round tip or cut about a 1/2cm off the corner.
Pipe 2.5cm (1") circles on the baking paper, leaving about 2cms between each.
Leave the macarons on your counter to form a skin, this should take 15-30 minutes depending on the weather. You'll know the skin has formed when you touch it and it doesn't stick to your finger.
Preheat the oven to 150c and place in the oven for 18-20 minutes.
Remove from the oven and let them cool completely on the tray before removing. Gently peel them off the baking paper to remove.
Fill biscoff into a piping bag and cut the corner of. Pipe small mounds into the centres of half the shells. If the biscoff is too firm to squeeze out you can place in the microwave for a few seconds at a time.
Top with the remaining shells and serve.
Can be stored in the fridge for up to 2 weeks.