Special Equipment: 10 (8-ounce) Ball jars
Make the Cake: Preheat the oven to 350 degrees F. Place 10 jars on a cookie sheet pan, evenly spaced out.
In a mixer fitted with a whisk attachment (or using a hand mixer), cream the butter until smooth. Add the sugar and eggs and mix until fluffy, about 2 minutes. Add the vanilla, cocoa, baking soda, and salt and mix. Add half of the flour; then half of the sour cream, and mix. Repeat with the remaining flour and sour cream. Drizzle in the coffee and mix until smooth. (The batter will be thin.)
Pour the batter into the jars, filling a little more than half way and bake until the tops almost firm to the touch, about 25 minutes.
While the cake is baking, make the Frosting: Melt the butter in a small saucepan over medium heat; then add the brown sugar and 2 tablespoons water and stir until the sugar is dissolved, for 2 to 3 minutes. Remove from heat and beat in the powdered sugar adding 1 or 2 tablespoons more water so that the frosting is fairly liquid and can slide down the sides of the cake in the jar.
While the cake is still warm, sprinkle the top with pecans and pour the brown sugar icing over to cover and soak in. Let cool then screw on jar lids and keep at room temperature until ready to eat.