Put oven rack in middle position and preheat oven to 350°F Lightly brush 8 muffin cups with some of melted butter and chill 2 minutes, then butter again and chill 1 minute more. Dust cups with matzo cake flour, knocking out excess.
Beat together softened butter, 2/3 cup sugar, and 1/8 teaspoon salt in a large bowl with an electric mixer at medium-high speed until pale and fluffy, then add egg yolks, 1 at a time, beating until well blended. Beat in lemon juice and 2 teaspoons zest until combined. Add flour and mix at low speed until just combined.
Beat whites with remaining 1/8 teaspoon salt in another bowl with cleaned beaters until they hold soft peaks. Add 2 tablespoons sugar, a little at a time, beating, then beat until whites just hold stiff peaks. Stir one fourth of whites into yolk mixture to lighten, then fold in remaining whites gently but thoroughly. Spoon batter into 8 prepared muffin cups.
Blend remaining 2 tablespoons sugar and 1 teaspoon zest with your fingertips and sprinkle over batter, then bake until cakes are puffed, edges are golden, and a wooden pick or skewer inserted in center of cake comes out clean, 20 to 25 minutes. Cool cakes in pan on a rack 15 minutes, then lift out cakes carefully (tops will break easily) and cool completely on rack.
Bring all syrup ingredients to a boil, covered, in a 3-quart heavy saucepan over moderate heat, stirring occasionally, then remove lid and boil 10 minutes. Pour syrup through a sieve into a bowl, pressing on and then discarding solids. Cool to room temperature.
Beat cream in a bowl with cleaned beaters until it just holds stiff peaks.
Spoon 1/4 cup syrup into each of 8 shallow bowls and top with cakes. Spoon whipped cream on top of cakes.