I use DARK unsweetened cocoa powder and DARK chocolate syrup (although the sugar-free chocolate syrup doesn't come in dark)
Just add 1 HEAPING TB. of unsweetened dark cocoa powder, plus 2 tsp. of chocolate syrup and 1 tsp. of INSTANT COFFEE powder to the cake or brownie mix when you prepare it. The coffee will not flavor the cake, but just intensify the chocolate flavor.
For canned chocolate frosting: For about each 1/2 cup of frosting that you use... I put it into a small bowl and stir/whisk in 1 tsp. unsweetened cocoa powder, plus 1/2 tsp. instant coffee powder... and then 2-3 tsp. of the chocolate syrup. You want the frosting to be a little more liquid and creamier than it is in the can anyways.