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1
Put oven rack in middle position and preheat oven to 400F.
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2
Butter a 10-inch glass or ceramic pie plate or a 2 1/2-quart oval gratin dish and chill.
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3
Toss rhubarb with brown sugar in a bowl until coated.
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4
Whisk together flour, baking powder, and 1/2 cup granulated sugar in a large bowl until combined well.
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5
Blend in butter with a pastry blender or your fingertips until mixture resembles coarse meal.
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6
Whisk together milk, whole egg, and yolk.
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7
Make a well in center of flour mixture and add milk mixture, stirring with a wooden spoon to gradually incorporate flour and form a soft, sticky dough.
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8
Transfer half of dough to chilled pie plate and pat out over bottom and halfway up side with well-floured hands, then spoon rhubarb and any juices onto dough.
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9
Using a tablespoon, spoon remaining dough in small mounds evenly over top.
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10
Lightly beat egg white with a few drops of water, then lightly brush cake with egg wash. Sprinkle remaining 2 tablespoons granulated sugar over top.
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11
Bake cake until top crust is golden and rhubarb is tender, 30 to 40 minutes.
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12
Transfer to a rack to cool, about 30 minutes.
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13
Beat cream with confectioners sugar, whiskey, and vanilla in a bowl with an electric mixer until it forms stiff peaks.
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14
Serve cake warm or at room temperature with whiskey cream.
