Preheat oven to 375°. In a microwave, melt 2/3 cup chocolate chunks; stir until smooth. Cool slightly. In a large bowl, cream butter and confectioners' sugar until light and fluffy. Beat in sweet potatoes, cooled chocolate, ginger and vanilla. In another bowl, whisk flour, cornstarch, baking cocoa, baking powder, baking soda and 1/4 teaspoon salt; gradually beat into creamed mixture., Shape dough into 3/4-in. balls; place 2-1/2 in. apart on parchment-lined baking sheets. Flatten slightly with bottom of a glass dipped in confectioners' sugar. Bake until edges are firm, 8-10 minutes. Remove from pans to wire racks to cool completely., Meanwhile, whisk cream cheese, dulce de leche, sweetened condensed milk, cinnamon, allspice and remaining salt until smooth. Spread filling on bottoms of half of the cookies; cover with remaining cookies., For chocolate coating, microwave remaining chocolate chunks; stir until smooth. Dip cookies halfway into chocolate; let stand until set. Store between pieces of waxed paper in an airtight container in the refrigerator.