1 c. unsalted butter, soft
1 c. plain yogurt
6 lg. egg whites, at room temperature
Pinch of cream of tartar
1 1/2 c. blueberries
2 c. cake flour, sifted with 1 tsp. baking soda and 1/4 tsp. salt
2 c. sugar
6 lg. egg yolks
4 tsp. grated lemon rind
2 tbsp. fresh lemon juice
1 tbsp. confectioners' sugar tossed with 1 tbsp. flour

In a large bowl, cream the butter, add 1 1/2 cups of the sugar, a little at a time, and beat the mixture until it is light and fluffy. Beat in the egg yolks, one at a time, beating well after each addition. Add the lemon rind and lemon juice, and stir in the flour mixture alternately with the yogurt.

In a bowl with an electric mixer, beat the egg whites with the cream of tartar and a pinch of salt until they hold soft peaks, beat in gradually the remaining 1/2 cup sugar, and beat the meringue until it holds stiff peaks. Stir in 1/4 of the meringue into the yogurt mixture and fold in the remaining meringue gently but thoroughly.

Fold in the blueberries, spoon the batter into a buttered and floured 4-quart tube pan, 4 inches deep, and smooth the top. Bake the cake in a preheated 375 degree oven for 1 hour or until a cake tester inserted halfway between the center and the edge comes out clean. Let cool in pan for 10 minutes, invert it onto the rack, and let it cool completely. Sift confectioners' sugar over the cake and transfer the cake to a platter, or serve sliced, toasted lightly and buttered.