Place half of the bread flour and sugar in a large bowl. Sprinkle the yeast into the center and pour the warm mushroom soup on top. Mix well with a spatula.
Add the remaining bread flour, powdered mushroom stock and butter and mix well until the butter is fully dispersed.
Transfer the dough to a floured surface and knead for 10 minutes.
Place the dough into a small bowl and cover with plastic wrap. Place this bowl into a larger bowl filled with some warm water. Let the dough rise for 40-50 minutes, until it has doubled in size.
Turn the dough out onto a floured surface and divide it into 8 pieces. Cover with a damp dish cloth and let rise for an additional 15 minutes.
Roll the dough into balls and place them into a lightly floured mini muffin tin. Fill the remaining empty muffin cups with boiling water and gently slide the tin into a plastic bag. Seal the bag closed and rise for a final 30-40 minutes.
Carefully remove the muffin tin from the bag and bake the bread at 375F for 15 minutes. Let cool in the pan for 5 minutes, then transfer the buns to a wire rack. Enjoy warm or cold.