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1
Preheat oven to 350u00b0F with rack in middle. Line bottom and sides of a buttered 15- by 10-inch shallow baking pan (1 inch deep) with a sheet of parchment paper and butter parchment.
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2
Melt butter with treacle, sugar, and water in a small saucepan over medium heat, stirring, then cool to room temperature, stirring occasionally.
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3
Whisk together flour, baking powder, and ground ginger in a large bowl. Whisk in butter mixture and egg yolk until smooth.
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4
Beat egg white in a medium bowl with an electric mixer until it just holds stiff peaks, then fold into batter gently but thoroughly.
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5
Pour into baking pan, spreading evenly, and bake until top is golden and firm to the touch, 18 to 22 minutes. Cool cake in pan 5 minutes, then cover cake with a slightly damp kitchen towel and cool completely, about 1 hour.
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6
Dust cake generously with confectioners sugar, then cover with a sheet of parchment, then with a baking sheet or rack. Invert cake and peel off parchment now on top.
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7
Beat cream until it just holds stiff peaks, then spread evenly over top of cake, leaving a 1/2-inch border. Sprinkle crystallized ginger over cream. Roll up cake, starting from a short side, to make a 10-inch log, removing parchment as you go.
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8
If desired, slices of roulade can be frozen (up to 1 week), then lightly sprinkled with turbinado sugar (such as Sugar in the Raw) and broiled (do not thaw first) in a shallow baking pan 5 to 6 inches from heat until golden, about 3 minutes.
