Heat oven to 350 degrees F. Turn muffin tin with 12 cups (or 2 tins each with 6 cups) upside down. With cooking spray, very generously coat bottom of every other muffin cup.
Work together dough and cocoa powder with wooden spoon in large bowl until well blended. (If dough becomes too soft, refrigerate to firm, 10 minutes.)
Spoon 2 level tablespoons dough onto bottom of coated cups; press evenly over sides, covering whole outside of cup.
Bake in 350 degree F oven 12 to 14 minutes, until crisp around edges (cookie will spread around base of cup, forming irregular collar). Remove pan to rack; cool for 5 minutes. Very gently loosen cookies with thin metal spatula; remove cookies to rack to cool. When muffin pan cools, repeat with remaining dough, shaping cups.
Combine white chocolate and cream in small saucepan. Heat over lowest heat, stirring, until white chocolate is melted and mixture is smooth. Stir in vanilla. Pour into bowl; cover with plastic wrap. Refrigerate, stirring occasionally, 2 hours, until very cold. (To speed chilling, place bowl of chocolate in larger bowl filled with ice cubes and water, being careful not to get water into mixture; stir occasionally.)
Beat chilled mousse on high speed until stiff peaks form, about 2 minutes. Stir in 1 cup chips, if using.
Spoon 1/3 cup mousse into each cup. If serving immediately, garnish with remaining mini chips if using. (Cups may be filled and refrigerated overnight. Garnish before serving.) Makes 14 filled cups.