This effect was achieved completely by accident! I didn't expect the incredible peeled paint effect this technique would create!
I took a piece of heavy brown board and spread Liquitex Modeling Paste thinly over the entire card with a wood craft stick.
I then smoothed the paste evenly over the entire card. If you want to paint over the morter before embossing, do it now.
Then, I let it dry for 30-45 minutes (40 to 50 minutes if you painted it). You don't want it to be completely dry - but just dry on the top. Actual time would depend upon the humidity, and how thickly you spread the paste. Touch the top of the card and push down slightly, and if it feels fairly dry, you can move on to the next step.
Place the card between an embossing folder. I used a Cuttlebug folder. Be sure you have placed the card in the folder to result in the effect you want top - raised or indented. Close the folder.
I then run the card through my Big Shot. I really think you could do also this with a rolling pin using lots of pressure.
Carefully remove the top of the folder, the part over the morter. Some of the mortar should peel off the card, leaving you with a very realistic peeled paint effect.
IMPORTANT: WASH THE EMBOSSING FOLDER THOROUGHLY. I used a toothbrush to get all of the mortar out of the cracks.
Let the card dry thoroughly before using. And here's what it looks like: