Grease and flour a bundt pan. Pour the pecans in the bottom. Preheat the oven to 350F.
In a standup mixer with a paddle attachment. Cream butter and sugar together, add eggs and vanilla and mix until smooth. Whisk flour, cocoa, baking soda and salt together. Add to the batter alternating with the cooled melted chocolate. Add the sour cream and half and half. Mix well scraping down the sides as needed.
Bake for 50-60 min. until a cake tester comes out clean.
Just before serving, melt the marshmallows, cream and butter in a microwave safe dish on half power for 2-3 min., stirring at 1 min. intervals until melted and smooth. Pour over the bundt cake and serve. Serve with vanilla ice cream if you like also!!
Nutrition calculation is an estimation only. If you need nutritional calculations for medical reasons, please use a source that you trust.