Preheat the oven to 400 degrees.
Wash and dry mason jars, set lids aside. Pint size jars work great. Spray with non stick spray.
For the cake: Sift together flour, unsweetened cocoa powder, and salt; whisk to combine.
In a large mixing bowl beat butter and sugar until creamed and combined.
Add eggs and vanilla and mix well.
In a small bowl or measuring cup mix buttermilk and red food coloring.
With the mixer running on low alternately add 1/4 of the flour and 1/3 of the buttermilk mix until fully combined (start with flour).
Finally mix baking soda and white vinegar in a small bowl or cup.
Pour into batter and mix well (if making batter ahead of time wait to do this step until right before you bake).
Pour batter into jars, filling halfway. I use a measuring cup for less mess.
Place jars on a baking sheet, 6 to 8 per baking sheet.
Bake for 20-30 minutes, depending on what size jars you use. Stick a toothpick or butter knife in the center and if they come out clean the cake is done!
Meanwhile, while cakes are baking, place lids (not rings) in a small saucepan filled with water. bring to a boil and then turn off heat and let sit in hot water while cakes bake.
As soon as cakes come out of the oven remove lids one out of a time with tongs and dry off with a hand towel.
Place lids on top of hot jars (wear oven mitts!) and screw the rings on each jar tightly.
Set aside to cool. The lids will pop when they seal from the heat, approx 30 min.
You can decorate with ribbon and a gift tag and don't forget to send a jar of frosting!