preheat oven to 180° C/350° F or 160° FF
In the bowl of a stand mixer, cream the butter and sugar. 1-2 minutes on medium-high speed is enough (you don't want too much air in the mixture)
Add vanilla, golden syrup and egg yolk, then mix until combined.
Add combined sifted dry ingredients and mix on low until just combined
Using a small ice cream scoop, scoop out the dough and roll into balls
Place dough balls onto a lined baking sheet, leaving space for them to spread a little
Bake for 8-10 minutes, until lightly golden
Cool for a few minutes, then transfer to a cake rack. When completely cool, you can fill them with the chocolate ganache
When the ganache is thick enough to hold it's shape, briskly stir it with a spatula, then pour into a piping bag fitted with a round (alternatively, just snip the end off a piping bag).
Pipe a generous amount of ganache onto the underside of one cookie, pressing together with a second cookie to sandwich