Melt butter and shortening in a large saucepan. Add boiling water, stir in molasses, then add baking soda and vanilla.
This mixture will foam, so make sure you have room for expansion. Let mixture cool slightly, then add sugar, eggs and raisins (if desired). Mix flour and rest of dry ingredients together in a large bowl. Stir in wet ingredients until just combined. Chill dough for 20 minutes. Preheat oven to 325°F. Turn dough out on a floured board and knead 8 to 10 times. Pat or roll out the dough to 3/4" to 1" thickness, cut into desired size. Place buns on a parchment lined baking sheet, and bake for 20 to 24 minutes. Buns will be a little soggy.