In a large bowl combine flour and sugar, in a small saucepan melt butter, add cola and cocoa, heat to boiling - cool slightly. Pour over flour and sugar. Stir until blended. Add buttermilk, eggs, soda & vanilla and mix well. Fold in marshmallows and pour into prepared pan Bake for 40 minutes. Cool
GLAZE: In a small saucepan mix butter, cocoa and cola - heat to boiling. Pour over sugar & mix well. Add pecans and pour over slightly cooled cake.