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Preheat the oven to 350°F. Lightly grease (or line with parchment) a 9" x 13" pan.
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To make the squares: In a medium-sized mixing bowl, whisk together the flour, ginger, allspice, salt, baking soda, and crystallized ginger.
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In a separate bowl, stir together the molasses, eggs, brown sugar, and butter.
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Combine the wet and dry ingredients, beating until smooth.
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Spread the batter in the prepared pan, and set it aside.
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To make the streusel topping: Using a pastry blender, electric mixer, or your fingers, mix together the flour, butter, salt, and brown sugar until it's fairly well-blended; some chunks of butter can remain. Mix in the crystallized ginger.
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Bake the squares for 15 minutes. Sprinkle on the streusel, and bake an additional 30 minutes, until the streusel is a deep, golden brown.
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Remove the squares from the oven, and run a knife around the edges of the pan to loosen them. Allow them to cool for 20 minutes, then cut into 2 1/4" squares.