In a large bowl, mix together butter and 1 cup sugar until fluffy. Add egg and vanilla.
Mix together flour, cocoa and salt in small bowl, then add gradually to sugar/butter mixture. Mix until combined.
If dough is sticky, refrigerate for 20 minutes or until dough is easy to handle.
Place ½ cup sugar in small bowl.
Roll heaping teaspoons of dough into balls about 1 inch in diameter. Roll each ball in the sugar and place about an inch apart on parchment lined cookie sheet.
Bake for 9 minutes, remove from oven then press peppermint kiss into each cookie.
Return to oven for 1-2 minutes until cookie is set and kiss slightly melted.
Let cookies stand on baking sheet for 1-2 minutes before removing to rack to cool thoroughly.