Firstly, the "highlight reconstruction" module doesn't actually adjust the exposure of hilight areas itself, it only determines how to handle areas that are clipped in the RAW file when they're pulled down to a level that doesn't result in clipping in the output. You can use the very useful RAW overexposure and output overexposure warnings available in the bottom panel of darkroom mode to check both conditions.

For doing the actual selective exposure adjustment, you can use the "shadows and highlights" module to do this based on the existing values, possibly combining with a drawn mask for greater selection control. For complete manual control of the process, you could use a drawn mask with the basic "exposure" module, which resembles traditional dodging/burning. If the overexposure is primarily in the sky or otherwise varies across the image, you might find the "graduated density" module useful. Sometimes a combination of these methods can work well.

UPDATE: That's the general answer; after looking at the image that you added to the question, I see that a light source next to a shiny object has completely blown out a large area. Since there isn't much information there to work with, you'll probably need to use the "color reconstruction" module, which can actually synthesize color information based on the area surrounding the blown highlights. Try starting with these values:

• threshold: 100
• spatial extent: 160
• range extent: 5

and adjust from there.