In a bowl, beat the egg whites with the cream of tartar until stiff. Gradually beat in 1/2 cup (110g) sugar.
In another bowl, stir together the flour, baking powder and salt.
In a large bowl, cream the butter with the remaining 1/2 cup sugar. Beat in lemon rind, lemon juice, sour cream and vanilla. Stir the flour mixture into the butter mixture along with about 1/3 of the beaten egg whites. Fold in the rest of the egg whites gently but thoroughly. Stir in the almonds. Turn the batter into a greased and floured ring tin.
Bake at 180 degrees C for about 70 minutes, or until it tests done with a toothpick. Cool on a wire rack.