Heat oven to 350. Butter and flour a tube cake pan. Combine the flour, salt, baking powder and soda. Next, beat butter, sugar and vanilla until fluffy, about 5 minutes. Add eggs one at a time beating after each one. Beat in flour mixture and yogurt alternatively. Remove 1 cup batter and stir in espresso (coffee). Pour 1/2 the plain batter into pan. Drop spoonfuls of the coffee batter on top. Add remaining plain batter. Stick a spoon handle into the pan all the way to the bottom and swirl batters. Bake until a toothpick comes clean, about 1 hour and 15 minutes.