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1
Combine rhubarb, sugar, and 1 tablespoon water in medium saucepan.
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2
Stir over medium heat until sugar dissolves.
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3
Cover; cook until rhubarb softens to thick pulp, stirring often, about 8 minutes.
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4
Mix in jam; cool.
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5
do ahead Can be made 1 week ahead.
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6
Cover; chill.
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7
Sift flour, sugar, baking powder, and salt into large bowl.
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8
Using large holes on box grater, grate frozen butter into bowl; stir to blend.
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9
Cover and freeze at least 15 minutes.
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10
DO AHEAD: Can be made 1 day ahead.
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11
Keep frozen.
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12
Preheat oven to 400F.
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13
Line large rimmed baking sheet with parchment paper.
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14
Add 2/3 cup ice water to flour mixture, tossing until soft dough forms, adding more ice water by tablespoonfuls if dough is stiff.
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15
Transfer to lightly floured surface; knead gently 5 or 6 turns (butter should remain in firm pieces).
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16
Roll out dough to 12x9-inch rectangle.
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17
Spread 6 tablespoons filling over dough, leaving 1-inch plain border on all sides.
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18
Starting at 1 long side, roll up jelly-roll style; seal seam.
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19
Place seam side down on prepared sheet.
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20
Seal ends.
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21
Brush glaze over pastry.
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22
Bake pastry until golden (jam may leak out), about 30 minutes.
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23
Cool pastry briefly.
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24
Cut roly-poly crosswise into 6 to 8 rounds.
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25
Serve warm with Vanilla Custard Sauce and remaining filling.
