For crust: With an electric mixer, beat butter and cream cheese until smooth. Stir in flour until dough forms. Transfer dough to floured surface and divide into 24 pieces. Press each piece into bottom and sides of an ungreased mini muffin tin (1½-inch muffin cups) to form little tart crusts. For filling: Combine apples, maple sugar, cornstarch and Baking Spice in a medium saucepan and cook, stirring occasionally, until mixture has thickened. For topping: Combine flour, brown sugar and nuts in a small bowl. Cut cold butter into small pieces, add to flour mixture and rub together with fingers until mixture is crumbly. To assemble: Place about 1 tsp. of filling into each dough-lined muffin cup and sprinkle each with topping. Bake in a 325-degree oven for about 30 min. until pastry is golden. Remove from oven and cool in muffin tin for 5 min., then remove tarts from tin and cool completely on a baking rack before serving.