Place the currants in a small bowl and pour the boiling water and lemon juice over them. Set aside to let the currants plump and cool, about 15-25 minutes.
In a medium bowl, stir together the flour, polenta, salt, vanilla powder and baking powder. In another bowl, lightly beat the egg and egg yolk and combine them with the melted butter and lemon zest. Stir the egg mixture into the dry ingredients using a wooden spoon to form a wet dough. Save the egg white you got the yolk from, that will be used for finishing the cookie.
Drain any liquid from the currants and stir them into the dough. Wrap the dough in a piece of plastic wrap and let it chill for one hour.
Roll your dough out to 1/4 inch thick sheets. We use a square cutter and cut it diagonally to get the triangle shape. Preheat the oven to 325°F and line a cookie sheet with parchment paper. Then you can lightly beat your egg white and brush the cookies very lightly. you don't have to cover it all, its for the sugar to stick. Sprinkle cookies with sugar.
Bake the cookies for 15 minutes or until they are golden brown and firm. Cool.