To make the cookies, cream the butter with the sugar in a large bowl until light. Beat in eggs and vanilla; set aside. Sift the cocoa, flour, salt, baking powder, and baking soda together.
Stir the dry ingredients thoroughly into the butter mixture. Cover the bowl with plastic wrap and refrigerate for 2 or 3 hours. Preheat the oven to 375 F and butter 2 cookie sheets.
Using a teaspoon measure, roll level teaspoons of the dough into balls and place them 2 inches apart on the sheets. Bake for 10 to 15 mins, until cookies are puffed and surfaces slightly dry and cracked. Cool on racks.
To make the filling, cream the butter until light. Beat in the vanilla and powdered sugar, adding whipping cream and continuing to beat until the consistency is like creamy frosting.
Makes about 3 dozen sandwiches.
Variation : For half a batch of vanilla-filled and half a batch of peppermint-filled cookies, add ⅛ teaspoon peppermint extract to half the filling. Tint the peppermint filling pink or green before filling half the sandwiches.