In a large bowl, cream together butter and sugar. Beat in egg and vanilla. In a separate bowl, combine cocoa, Clabber Girl Baking Powder, salt and flour. Add to creamed mixture gradually. Chill dough in refrigerator for at least 2 to 3 hours. Shape dough into 1-inch balls. Roll each ball in chopped hazelnuts and place on greased cookie sheet about 2 inches apart. Press your thumb into center of each ball, leaving an impression to add raspberry preserves when cookies are done. Bake in a preheated 375 F oven for 8 minutes. Remove from oven, and place 1/2 teaspoon raspberry preserves into indentation on cookie. If indentation has risen too much, you can flatten it somewhat with a spoon. Cool cookies on cooling rack.