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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.
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Fold in the vanilla and melted butter. Cook in a mini-pizzelle iron according to the manufacturer's directions.
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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.
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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).
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Remove the saucepan from the heat, and allow the mixture to cool for about 10 minutes.
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Place one teaspoonful of filling (more for larger cookies) onto a cookie.
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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.
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Store the cookies in a cool, dry place for 1 week, or freeze them for 1 month.