1. Whisk together flour, soda, baking powder and salt in a bowl.
2. Beat butter, oil, brown sugar and sugar until smooth in separate bowl.
3. Beat in egg whites one at a time and then beat in coffee and vanilla extract.
4. Add 1/4 cup yogurt and beat until smooth.
5. Add half of flour mixture and beat until smooth.
6. Beat in remaining 1/4 cup yogurt, then remaining flour mixture.
7. Beat 2 to 3 minutes, or until batter is smooth and creamy.
8. Spread batter in a greased 9 inch round cake pan and bake in a preheated 325F oven for 35 to 40 minutes, or until toothpick inserted in center comes out clean.
9. Cool 10 minutes.
10. Meanwhile, chop the chocolate bars into small pieces.
11. It`s fine if some pieces are a bit larger than others.
12. Unmold cake from pan, and set on wire rack. Sprinkle chocolate-toffee bits on top of cake while hot. Cool completely before serving.
13. While its cooling, you will see that the toffee bits on top are slowly melting on the surface of the cake.
14. Cut them into slices and serve warm.

Courtesy: http://chefinyou.com/