Line 2 baking trays with baking parchment. In a bowl, beat the butter and caster sugar together until soft and creamy. Stir in the Caramel Flavour.
Sift the flour on top and mix well. Using your hands, bring the mixture together to form a soft dough. Turn on to a lightly floured surface and knead gently to make sure the mixture is well blended – this mixture is quite short.
Divide the dough into 4 equal portions. Flatten each portion slightly and add a few drops of one Gel Food Colour to each. Fold the dough over the Gel to enclose it and carefully knead the dough until evenly mixed - you may need to dust your fingers with flour if the dough becomes sticky. Keep adding Gel in this way until you have achieved the desired shade – as a guide, we used: 40 drops Blue; 40 drops Pink; 50 drops Green and 50 drops Yellow.
Lightly dust the work surface with flour and roll out each piece thinly to form a rectangle approx. 20 x 24cm (8 x 10inch). Brush each piece lightly with a little water and carefully lay all the colours on top of each other, pressing gently to stick them together.
Starting from one of the shorter sides, carefully roll up like a Swiss roll. Because the mixture is short, it will crack but if you roll the rolled up mixture gently back and forth, this will help smooth the surface. Wrap and chill for 30 minutes.
Ten minutes before baking, preheat the oven to 180°C (160°C Fan oven, 350°F, Gas 4). Make up the Egg White Powder as directed on the sachet. Unwrap the roll and brush all over with egg white. Mix the 4 colours of the Bright and Bold 4-cell mix together and sprinkle over the roll, turning to make sure the roll is completely covered.
Slice into 16 rounds (approx. 1cm (1/2 inch thick) and place on the prepared baking trays. Bake for 12-15 minutes until very lightly browned. Leave to cool on the baking tray for 5 minutes before transferring to a wire rack to cool completely. Your cookies are now ready to serve and enjoy!