Babbitt, Minnesota where a frigid morning low temperature of -46 degrees was recorded on January 21, 2011.
From NWS Duluth, MN, an old record beaten by five degrees:
Record weather event
National Weather Service Duluth, Minnesota

5:18 PM CST. Fri Jan 21, 2011

...Record Low Temperature Set At International Falls, Minnesota...

A record low temperature of -46 degrees F was set at International Falls, Minnesota today. This breaks the old record of -41 F set in 1954.
Here is a map (above) of a few notable low temperatures across the area. The contoured shading in the background was created by interpolating the observations using GIS software. Therefore, the contours may not necessarily match individual observations, but the overall trend in temperatures is pretty close. Areas near Lake Superior managed to stay much warmer than outlying areas, and you can even detect a hint of urban heat influences around the Twin Cities.

These pictures are from Babbitt, Minnesota where a frigid morning low temperature of -46 degrees was recorded on January 21, 2011. The pictures were taken by observer Ryan Scharber. The pictures show ice fog, which is usually a shallow fog consisting of suspended ice crystals. Ice fog usually only occurs when temperatures fall below -22 degrees Fahrenheit (-30 Celsius) according to the AMS Glossary. Normally, fog consists of tiny water droplets, or supercooled water droplets. However, when temperatures are as cold as they were this morning in parts of northern Minnesota, it becomes too cold for liquid water to exist, and small ice crystals can develop if the amount of water vapor in the air is sufficient.

Some notes of interest:
  1. The lowest temperatures recorded in the NWS Duluth county warning area were -46 degrees at both International Falls, MN (ASOS) and Babbitt, MN (CO-OP).
  2. The -46 degree low was tied for the 5th lowest on record at International Falls. Temperature records date back to 1897. The record is -55 degrees which was recorded on January 6, 1909.
  3. The -46 degree low was tied for the lowest on record at the International Falls Airport. The official observing station was moved to the airport in 1939. This is tied with the -46 degree reading from January 6, 1968.
  4. The -25 degree low at Duluth is tied for the 5th lowest minimum temperature in the last decade (since 2000). The lowest minimum temperature of the 2000s thus far has been -30 on January 29, 2004.
  5. The state record low temperature in Minnesota was recorded at Tower, Minnesota on February 2, 1996. The low was -60 degrees. That was also the coldest temperature ever recorded east of the Mississippi River.
  6. The state record low temperature in Wisconsin was recorded at Couderay on February 2nd and February 4th of 1996. The temperatures dipped down to -55 degrees both nights.
List Of Coldest Morning Lows for Minnesota, Friday January 21, 2011 available here

BTW, International Falls holds the registered trademark of Icebox of the Nation

Source: NOAA