Preheat the oven to 180°c (160 fan/ Gas 4/ 350F) and line 2 baking sheets with baking paper.
Cream together the butter and sugar until light and fluffy.
225 g Unsalted Butter, 100 g Golden Caster Sugar
Beat in the golden syrup, vanilla extract, and boiling water.
2 teaspoon Vanilla Extract, 1 tablespoon Golden Syrup, 1 tablespoon Boiling Water
Mix in the plain flour, baking powder, bicarbonate of soda, and rolled oats.
225 g Plain Flour, ½ teaspoon Baking Powder, ½ teaspoon Bicarbonate of Soda, 75 g Rolled Oats
Scoop heaped teaspoons of dough and roll them in your hands to form small balls. Place on the lined baking trays and flatten slightly with the palm of your hand. Bake in the oven for 15 minutes.
Beat together softened butter, vanilla extract and sieved icing sugar until you get a smooth lump-free buttercream.
75 g Salted Butter, 150 g Icing Sugar, 1 teaspoon Vanilla Extract
Once the biscuits are completely cool spread a thin layer of buttercream over the flat base of half of the biscuits and sandwich together with the remaining biscuits (so both flat bottoms of the biscuits are touching the buttercream).