Milk doesn’t turn into ice cream when it’s cold, but that’s an old joke. “It was so cold that when I milked old Betsy I got ice cream,” a 1959 newspaper printed.
             
Chocolate ice cream is sometimes included with the joke. A “brown cows” version has been cited in print since at least 2003:
   
“It was so cold…when we milked the cows, we got ice cream! When we milked the brown cows—we got chocolate ice cream!”
     
   
19 February 1959, Alton (IA) Democrat, “Did you ever stop to listen?.”  pg. 3, col. 7:
That led us to thinking about the conversation we heard about cold weather, the best heard was: “It was so cold that when I milked old Betsy I got ice cream.”
   
Athropolis (December 4, 2003)
It was so cold…
(...)
17. It was so cold . . .
when we milked the cows, we got ice cream! When we milked the brown cows - we got chocolate ice cream!
 
September 2005, Bulletin of the American Meteorological Society (Boston, MA), “Nowcast: Papers of Note,” pp. 1217-1222:
AccuWeather.com meteorologist Henry Margusity’s weather humor for a cold winter’s morning goes a little something like this: It was so cold this morning . . . How cold was it?!? . . . When the farmers went to milk their cows, they got ice cream!
 
13 November 2005, Palm Beach (FL) Daily News, “Climate’s more agreeable with humor” by Mike Lyons, pg. D2, col. 3:
It was cold this morning. How cold was it? It was so cold that when the farmers went to milk their cows, they got ice cream.
   
Google Groups: alt.jokes
So Cold Jokes
John
1/16/07
It’s soooo cold…
(...)
when we milked the cows, we got ice cream! When we milked the brown cows - we got chocolate ice cream!
 
17 February 2007, Asbury Park (NJ) Press, “Today’s Weather: More of the same frio” by Fred Simmonds, pg. ?:
It was so cold a farmer went out to milk his cows and got ice cream instead.
   
South Marion Citizen (Dunnellon, FL)
Some of my best friends are (snow) flakes
By Dick Frank
Thursday, December 18, 2008 at 7:00 pm
(...)
When we milked the cows we got ice cream. When we milked the brown cows we got chocolate ice cream.
   
Twitter
Jean Hanson‏
@jeanhanson
@DaveLetterman Thanks for the follow! It was so cold in MN last night the farmers got ice cream when they milked the cows!
10:36 AM - 1 Jan 2009
 
SkyWriting.Net (January 25, 2009)
How Cold Was It?
[ Author Unknown—from Patricia, via ‘Buffalos Chips’ (buffalos-g-jokes.yahoogroups.com) ]
IT WAS SO COLD THAT . . . . .
(...)
15. When the cows were milked, we got ice cream! Milking the brown cows, we got chocolate ice cream!
     
Twitter
ISTC‏
@IntlSportsCamp
It’s so cold… when we milked the cows, we got ice cream! When we milked the brown cows - we got chocolate ice cream!
9:13 AM - 8 Dec 2009
   
Google Books
Maine Curiosities:
Quirky Characters, Roadside Oddities, and Other Offbeat Stuff (Third Edition
By Tim Sample and Steve Bither
Guilford, CT: Globe Pequot Press
2011
Pg. 30:
It was so cold that…
(...)
When they milked the cows, they got ice cream. The brown cows gave chocolate ice cream.
 
106.5 WYRK (Buffalo, NY)
10 FAVORITE ‘IT’S SO COLD’ JOKES
WENDY LYNN January 3, 2018
(...)
IT’S SO COLD…...
(...)
7. When we milked the cows we got ice cream