Preheat the oven to 180°C, fan 160°C, gas 4. Line 2 baking sheets with baking paper. Put the butter and sugar in a bowl and cream together using an electric handheld whisk. Beat in the vanilla.
Sift together the flour, cocoa powder and bicarbonate of soda, then add to the butter and sugar. Mix until crumbly, then, with clean hands, gather the dough and knead until just smooth.
Turn out on to a work surface and shape into a 27cm-long sausage shape. Slice into 18 rounds and place, spaced apart, on the baking sheets. Flatten each with your fingertips to a 5cm diameter. Bake for 10-12 minutes. Cool on the baking sheets for 10 minutes then transfer to a wire rack to cool completely.
To decorate, dust your work surface with icing sugar and roll out the white icing to a thin sheet. Use an 8cm-round cookie cutter to stamp out 9 circles. Cut each one to make two wavy-edged semicircles. Lightly brush with water and stick on to the cookies.
Roll out the green icing and cut out 36 leaf shapes. Use the tip of a teaspoon to cut away the edges to resemble holly leaves. Attach two to each cookie with a dab of water. Use the writing icing to dot two berries on each cookie.