Heat oven to 350º F. Coat (3) 8-inch round baking pans with cooking spray then lightly dust with flour. (Or use this excellent baking pan release recipe #78579).
Blend instant coffee with yogurt then add water, a few drops at a time, until coffee dissolves. Set aside.
Beat egg whites and cream of tartar until foamy. Gradually beat in 1/2 cup sugar until stiff peaks form. Set aside.
Whisk together the flour, baking powder, salt and nutmeg.
In a large bowl, beat butter until smooth and creamy. Beat in remaining 1 cup sugar until light in color, about 2 minutes.
On low speed, beat flour mixture, alternating with yogurt, into the butter and sugar.
Fold in half the egg whites to lighten then fold in remaining egg whites.
Scrape batter into prepared baking pans, spreading level.
Bake for 25 minutes, or until toothpick inserted in the center comes out clean. Remove from oven, let cool in pans for 10 minutes then invert onto a rack and cool completely.
Whip cream and confectioners' sugar to stiff peaks.
Once cakes have cooled, place 1 layer on a plate and spread with 3/4 cup whipped cream. Top with 2nd layer and another 3/4 cup whipped cream. Top with final layer & swirl remaining whipped cream on top. Sprinkle with more ground nutmeg, if desired. Refrigerate until serving time.