1) Preheat the oven to 180C/Gas 4. Line a baking sheet lined with parchment paper.
2) In a medium mixing bowl, beat the butter, sugar and vanilla with a hand mixer until smooth. Stir in the flour, cornmeal and lemon zest and beat until the dough clings together.
3) Divide the dough into 3 equal portions, roll into ropes about 2.5cm thick and place each rope on the lined baking sheet.
4) Flatten the ropes slightly and press the back of a small teaspoon, measuring spoon or your finger into each to make a 1cm-wide indentation for every 2.5cm of rope. Spoon a 1/4 teaspoon of jam into each indentation.
5) Bake the ropes in the oven for 10 to 14 minutes, until the sides just start to brown. Remove from the oven and allow to cool on the baking sheet for 5 to 10 minutes.
6) Meanwhile, place the chopped white chocolate into a plastic food bag in warm water. Push the chocolate into the corner of the bag until it has thoroughly melted.
7) Wipe off any water on the outside of the bag and snip the corner to drizzle chocolate over the ropes.
8) Chill until the chocolate has set. With a sharp knife cut the ropes diagonally into 2.5cm wide pieces. Store the biscuits in an airtight container, separating the layers with waxed paper.