Knead marzipan with 3/4 cup powdered sugar, chopped walnuts, rum, and maraschino liqueur on a clean work surface until smooth.
Sprinkle remaining powdered sugar on a clean work surface. Roll out marzipan and cut out circles, squares, or rectangles.
Line a baking sheet with waxed paper and set a wire rack on top.
Place 3/4 of the chocolate in the top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is melted, about 5 minutes. Remove from heat and stir in remaining dark chocolate until smooth.
Stick a toothpick or metal skewer into each praline and dip into the melted chocolate, letting excess drip off. Set onto the wire rack. Allow chocolate to set a little, then place 1 walnut half on top of the chocolate.
Allow chocolate to harden completely. Store pralines in airtight containers.