Preheat oven to 350 degrees. Generously grease a 9-by-13 inch baking pan or two 9 inch round cake pans. Set aside (I like bundt pans!) In a large bowl with an electric mixer, beat the egg yolks until they are pale yellow and fluffy. Add 1 1/2 cups sugar and beat to combine. Sift together the flour, cocoa, salt and baking powder. Add slowly to the egg mixture while beating. Stir in the chai concentrate and vanilla. In a medium bowl, beat the egg whites with the cream of tartar until foamy. Gradually beat in the remaining 1/2 cup sugar and continue beating until stiff, but not dry. Gently fold the egg whites into the cake batter, being careful not to deflate the whites. Pour into prepared pan and bake for 45 minutes for a 9X13 inch pan or 35 minutes for two 9 inch pans. A toothpick inserted in the center of the cake should come out clean. Cool completely before removing from the pans.