Preheat oven to 350°. Generously grease a 9x13 baking pan or two round cake pans. Set aside. In a large bowl, beat the egg yolks until they are pale yellow and fluffy. Add 1 ½ 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 Oregon Chai® and vanilla. In a medium bowl, beat the egg whites with the cream of tartar until foamy. Gradually beat in the remaining ½ cup sugar and continue beating until stiff peaks form. Using a spatula, gently fold the egg whites into the cake batter, being careful not to deflate the whites. Pour into prepared pan(s) and bake for 45 minutes for a 9x13, 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.