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 microwave. Grease a 12-cup muffin pan.
Roll dough into a long cylinder and then divide into 12 equal portions. Divide each of the 12 portions into 5 equal pieces and roll each piece into a ball.
For each bun, coat five balls with melted butter and then roll them in cinnamon sugar. Arrange four balls in the bottom of a prepared muffin cup. Using your fingertip, make a small well in the center and press in the fifth ball. Repeat with remaining dough and muffin cups.
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 350F.
Bake buns until golden, 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.
