Preheat the oven to 350F/175C. Line a baking sheet/tray with a silicone mat or parchment paper.
Place the butter and sugar in a bowl and beat/cream together (can use a stand mixer or hand mixer as you have/prefer, or even a wooden spoon). Cream until gently glossy.
Lightly beat the egg and add half in with the butter mixture and reserve the rest for later. Add the vanilla and golden syrup to the butter as well and mix all in so well combined.
Place the flour in a separate bowl and add the baking powder, salt and cocoa powder. Whisk together so the additions are well distributed through the flour.
Add the flour mixture to the butter mixture and mix in with spatula or mixer until combined but try not to overmix.
Take half of the mixture and form it into a log, lengthwise, on one side of the baking sheet/tray. The log should be roughly 14 inches (35 cm) long. Repeat with the other half of the dough on the other side of the baking sheet, leaving a little space between them and either side as they will expand a little as they bake. Gently flatten both logs so they are of an even thickness the whole way along and are roughly 1 ½ - 2 inches wide (4 - 5cm) and 0.4in (1cm) high.
Brush the top of both logs with the reserved egg, only leaving a thin layer of egg - you don't want too much as it will just dissolve the sugar. Sprinkle the pearl sugar over the top of each log then bake.
Bake the cookies for around 12 - 15 minutes until the top is dry and the outside feels gently firm. Allow to cool for a couple minutes then cut while still warm into slices on the diagonal, roughly ¾in (2cm) wide. Leave to cool a few more minutes before transferring to a cooling rack to cool completely, or diving in to enjoy.