Preheat oven to 350*F. Butter and flour a 9 to 10 inch bundt pan or tube pan and set aside.
To make the molasses batter, cream together the butter and brown sugar until very light. Beat in the egg yolks one at a time; beat in the molasses. Sift the dry ingredients together and add the batter alternately with the yoghurt, beginning and ending with the dry ingredients. Do NOT over mix.
To make the lemon batter, cream together the butter and the sugar and lemon zest until very light. Add the lemon extract. sift the dry ingredients together and add to the creamed mixture alternately with the yoghurt beginning and ending with the dry ingredients. Do NOT overmix. Beat the egg whites with the salt until nearly stiff. Fold about 1/4 of them into the batter to lighten it, then gently fold in the remainder.
Drop 3 or 4 large dollops of the lemon batter into the prepared cake pan. Top with 3 or 4 dollops of molasses batter. continue layering batters until all is used up. Gently rap the pan once against the countertop to settle the batters. With a butter knife, swirl the batters together a few times without over-doing it.
Bake until the cake is golden brown and a toothpick inserted near the centre comes out clean, about 1 hour.
Cool the cake in the pan on a wire rack for 10 to 15 minutes. Loosen the cake from the pan and unmold onto the rack to cool completely. Dust with icing sugar before serving. A lemon glaze is also nice on this.