Preheat the oven to 180C (350F). Place the hazelnuts in a small roasting tray and toast in the oven until they start colouring. Do not leave them in there too long, they just need to dry out slightly. Set aside until cool.
Place the hazelnuts and icing sugar in a food processor and blitz, in short bursts, to a fine powder.
Add the ground almonds and blitz to combine. Sift everything into a large bowl.
Pour half the egg whites into the bowl and mix together with a spatula until the mixture forms a paste.
Put the caster sugar and water in a saucepan. Put the rest of the egg whites in the bowl of your stand mixer.
Bring the sugar to the boil and clip on a candy thermometer. When the sugar temperature reaches 110C / 230F start whisking the egg whites on high speed. Keep a watchful eye on the sugar - once it reaches 117 C /242F pour it very carefully down the side of your mixing bowl while still whisking. Continue whisking for several minutes till the mixing bowl is cool. You will have a very glossy, stiff meringue. Add the pink colouring paste, if using, and mix it in.
Add the almond paste to the meringue bowl and mix it together until it's well combined. Make sure you scrape the bottom and sides of the bowl. The consistency of the batter is very important - it needs to flow smoothly and when dropped back into the bowl it should spread slowly.
Line 3 heavy trays with baking parchment - secure the paper by dotting a bit of macaron batter on each of the tray corners.
Put the batter into a piping bag with a 1 cm / ½-inch plain round nozzle. Pipe small rounds of batter onto the baking parchment, spaced slightly apart as the macarons will spread slightly. Use a template if you want absolute uniformity.
Carefully but forcefully knock the trays against the counter to release any air bubbles. If the macarons have little 'peaks' pat them down with a clean, wet finger.
If you are using the sprinkles, add them to the half the shells and leave them to dry out for 15-30 minutes.
Preheat the oven to 150C / 300F. Once the shells are touch dry, bake for 14-17 minutes. The macarons are ready when they have a smooth, dry top and are firm to the touch. Leave them to cool down completely before carefully lifting off the baking paper.
Stir the clotted cream until smooth and transfer into a piping bag fitted with a small round tip.
Pipe the clotted cream on half the shells, leaving space in the centre for a small dollop of jam.
Sandwich them gently together. Serve immediately.