Preheat oven to 350F with rack in middle. Generously
grease a 9- by
2-inch round cake pan with softened butter or vegetable oil and dust
with some matzo meal, knocking out excess.
Pour hot water over dates in a large bowl and let
stand 15 minutes
to soften. Pulse walnuts in a food processor until chopped, then add
1/4 cup sugar and pulse until nuts are finely ground. Add matzo meal
(2/3 cup), zest, cardamom, and salt and pulse until combined.
Beat egg whites with a pinch of salt in a bowl
using an electric
mixer at medium-high speed until they just hold soft peaks. Add
remaining 1/2 cup sugar in a slow stream, beating until whites hold
stiff glossy peaks. Whisk yolks into date mixture. Fold one third of
yolk mixture into whites, then fold in remaining yolk mixture gently
but thoroughly. Fold all of nut mixture into batter.
Spoon batter into cake pan and bake until golden
and springy to the
touch and cake just begins to pull away from side of pan, 35 to 40
minutes. Cool in pan on a rack 30 minutes, then invert onto rack and
cool completely. Serve with whipped cream.