CAKE: Butter and flour the bottom and sides of a 9 inch springform pan; set aside.
In a large bowl beat whites and cream of tartar to soft peaks.
Gradually beat in 1/4 cup of the sugar, continuing to beat to stiff peaks.
In a separate bowl beat the yolks with remaining sugar until light yellow and thickened.
Scrape yolks over whites, add lemon rind and vanilla and fold together.
Sift flour and salt over batter, folding in gently but thoroughly.
Transfer to prepared pan.
Bake at 350°F for 25 minutes or until the top springs back when lightly touched.
Let cool in pan on wire rack.
If making ahead, remove from pan and wrap well for storage at cool room temperature for one to two days or in the freezer for up to 2 months.
MOUSSE: In a very small saucepan sprinkle gelatin over water; set aside.
Rinse, hull and puree berries.
In a small saucepan combine puree, sugar and lemon juice and heat gently just long enough to dissolve sugar.
Remove from heat.
Warm softened gelatin over low heat until clear and syrupy.
Stir into strawberry mixture.
Transfer to a large bowl and chill to consistency of raw egg white.
Whisk in the yogurt.
Whip cream until form; fold into mousse and return to fridge.
SYRUP: In a small saucepan bring water and sugar to a boil.
Remove from heat, cool and stir in liqueur.
ASSEMBLY: Using a long serrated knife, cut cake into 2 thin layers.
Place top half, but side up, in the bottom of a clean 9 inch spring for pan.
Drizzle half the syrup evenly over the cut side of each of the 2 halves.
Spoon strawberry mousse over cake in pan; don't worry if mousse extends over the sides.
Set remaining layer, cut side over mousse, pressing gently.
Cover and chill thoroughly, overnight if possible.
Release sides of pan, and using a wide lifter, transfer cake to service plate, doily-lined if you've thought that far ahead.
CREAM FROSTING AND GARNISH: Rinse and hull berries.
Slice thinly.
Whip cream until it's stiff and firm.
Sweeten with sugar and spread evenly over top and sides of cake.
Arrange berry slices in circle around the top and bottom edges of the cake.
Refrigerate until serving time, which should be within the following hour or two.
