Heat oven to 180C/fan 160C/gas 4. Beat together the softened butter with the caster sugar until creamy. Stir in bicarbonate of soda, golden syrup and the egg yolks. Sift in the plain flour and ground cinnamon and ground ginger then bring together with a wooden spoon. Shape into two balls, knead until the dough comes together, then chill for 30 mins. Roll out one ball at a time, to about 2 x £1 coin thickness. Stamp out trees with a 7cm cutter, then re-roll the trimmings. Lift dough onto greased baking sheets and bake for 10-12 mins until slightly risen and golden brown. Cool on a wire rack. Will keep in an airtight container up to a week.