1. Preheat the oven to 400 degrees. Line baking sheets with parchment, or grease well.
2. In a large bowl cream butter and sugar together until fluffy. Add egg and vanilla, and mix until combined.
3. Add buttermilk, mix until combined.
4. Add flour, salt, and cocoa, mix until combined.
5. Dissolve baking soda in hot water. Add mixture to the batter, and mix until liquid is incorporated.
6. Place batter in piping bag or in large ziplock (cut off corner, about 1/2 inch). Create heart shapes by piping two V-shapes - use about 1 tablespoon of batter per heart.
7. Bake for 8 minutes, or until just set - should not be crispy.
8. Allow to cool for a few minutes on the baking sheet, then transfer to cooling racks.