To make the cookies: In a large mixing bowl, beat together the eggs, brown sugar and salt. In a medium-sized bowl, whisk together the flour, cinnamon and baking powder, then stir these dry ingredients into the egg mixture., Fold in the vanilla and melted butter. Cook in a mini-pizzelle iron according to the manufacturer's directions., To make the filling: Combine the water, sugar, butter and syrup in a medium-sized saucepan, stir over low heat until the sugar dissolves, then cover the pot and boil the mixture for 3 minutes., Add the cinnamon, and continue to boil until the syrup reaches "soft ball stage" (234°F to 240°F on a candy or instant-read thermometer)., Remove the saucepan from the heat, and allow the mixture to cool for about 10 minutes., Place one teaspoonful of filling (more for larger cookies) onto a cookie., Top with a second cookie, pressing together to spread the filling. Be careful, as the sugar syrup will be very hot. If the syrup cools and becomes hard to spread, rewarm it slightly., Store the cookies in a cool, dry place for 1 week, or freeze them for 1 month.