In a large mixing bowl, sprinkled the yeast over the water and let it stand until bubbly, 5 to 10 minutes. Stir in the butter, sour cream, sugar, egg, and vanilla.
Beat in 2 cups of the flour, the salt, and the baking soda until combined.
Add the remaining 1/2 cup flour and continue mixing until a smooth, soft dough forms.
Turn the dough out onto a lightly flour surface (keep your mixing bowl handy) and knead it until smooth, about 3 minutes. As you knead, add more flour if the dough is sticky, no more than 2 tbsp though. Place the dough back in the mixing bowl and cover with plastic wrap. Let the dough rise in a warm place until doubled in size, about 1 hour.
Lightly grease a 12-cup muffin pan, then flour the cups and tap out the excess.
In a small bowl, blend the sugar and cinnamon for the coating. Place the melted butter in a small dish and set aside.
Gently punch down the dough to deflate it. Form it into a long cylinder and divide it into 12 equal portions. Using kitchen scissors or a sharp knife, divide each of the 12 portions into 6 equal pieces, then roll each piece into a ball.
To make a bun, dip six of the balls in melted butter (be generous with the butter), then roll them in the cinnamon-sugar mixture. Arrange five of the balls in the bottom of a muffin cup. With your fingertip, make a small well and press the sixth ball in the center. Repeat with the remaining dough balls. Loosely cover the buns with plastic wrap and let them rise in a warm place until they’re puffy and double in size, about 40 minutes. Preheat oven to 350°F.
Bake the buns on the center rack until golden, about 20-22 minutes. Transfer the pan to a wire rack placed over a sheet of waxed paper or aluminum foil to cool for about 5 minutes. Now tilt the pan and gently tap it on the counter to release the buns. If needed, you can use a knife to release them from the pan. Place the buns on the wire rack.
In a small bowl, combine the ingredients for the glaze and stir until smooth. If the glaze is too thick, you can thin it out by adding 1 tsp water at a time. Brush each bun with the glaze. Serve immediately or at room temperature.