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Combine the flour, corn flour and baking powder together in a bowl with a pinch of salt. Using a food mixer or an electric whisk in another bowl beat the butter together with the sugar and lemon zest until very pale. Add the egg yolks followed by the Cognac and vanilla extract. Beat in the dry ingredients until you have smooth dough. Wrap the dough in clingfilm and chill for a minimum of 1 hour. You can make the biscuit dough the day before and leave in the fridge.
Line two large baking trays with baking parchment. Roll out the dough on a lightly floured surface to the thickness of a pound coin then cut out 60 biscuits with a 5cm round or fluted cutter. Put the biscuits back in the fridge for 20 mins to firm up.
Heat the oven to 180C/160C fan/gas 4. Bake the biscuits for 8 mins until just set. You want the biscuits to stay pale with a crumbly texture. Leave to cool completely before sandwiching two biscuits together with a spoonful of dulce de leche. Once all the biscuits are sandwiched together roll in desiccated coconut.