Place water in bowl of stand mixer and sprinkle yeast over. Let stand until bubbly, 5 to 10 minutes.
Stir in butter, sour cream, sugar, egg, pumpkin and vanilla. Beat in 3 cups of flour, the salt and baking soda until incorporated. Add remaining 1/2 cup of flour and continue to mix until a smooth, soft dough forms. If dough does not come together, add another 1/4 cup of flour. The dough should remain sticky and wet (you don't want to add too much flour and dry out the dough) but you should be able to remove entire dough from mixing bowl.
Knead dough until smooth, using the dough hook attachment, about 2-3 minutes. Place dough in a glass bowl greased with oil. Cover with plastic wrap. Let rise in a warm place until doubled in size, about 1 hour.
In a small bowl, mix cinnamon and sugar for coating. In another small bowl, melt butter in the microwave. Grease mini bundt pans. My pumpkin mini bundts are about the size of muffin pans. Roll dough into a long cylinder and then divide into 12 equal portions (less if you have bigger and fewer pans). Divide each of the 12 portions into 8 equal pieces and roll each piece into a ball.
For each bun, coat eight balls with melted butter and then roll them in the cinnamon-nutmeg sugar. Arrange 8 balls in each prepared muffin cup.
Cover buns loosely with plastic wrap and let rise in a warm place until puffy and have doubled in size, about 40 minutes.
Preheat oven to 350°F. Bake buns until golden and dough is fully cooked, about 20 minutes. Let cool in pan.
Place buns on a wire rack. Place a piece of parchment paper underneath wire rack. In a small bowl, mix glaze ingredients and stir until smooth. Drizzle tops of bread buns with glaze.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.