In a large bowl, stir together the brown sugar and butter until smooth and creamy. Add eggs and vanilla and beat well. Gradually stir in flour. Set bowl over a hot water bath and stir to blend ingredients. Cover and refrigerate overnight.
Preheat the pizzelle iron.
Shape dough into 3cm balls and place in the pizzelle iron. Close the iron and cook the biscuits. This should take about 30 seconds for an electric iron, or 2 to 4 minutes in a handheld iron over a medium flame. Carefully remove biscuits and cool on wire racks.