Preheat your oven to 180°C (350°F). Line a baking tray with baking parehment.
Melt the butter and syrup in a pan over medium heat, stirring.
Mix all the remaining dry ingredients in a bowl and then add the butter and syrup mixture. Mix well until combined.
Using your hands, roll the mixture into small balls about the size of a walnut. (You do need to work it together a bit - as if you are using a stress ball).Place on a baking tray and flatten with a fork. Reshape if necessary. (These biscuits do not spread much so flatten as much as possible).
Bake for 13 - 15 minutes. They will still be soft when you take them out of the oven but will firm up once cool.
For the chocolate filling
Chop the Mars Bar into small pieces and melt over a medium heat together with the cream and dark chocolate. Mix to a glossy sauce and then refrigerate until it is thick enough to spread on the cookies.
Build sandwiches by spreading a layer of chocolate filling on one biscuit and pressing a second one on top.