Special equipment: 2-by-2 1/2-inch heart-shaped cookie cutter
Add and sift the cocoa powder with the flour, baking powder and salt. Proceed with Sugar Cookie Dough recipe and chill for 2 hours.
Preheat the oven to 375 degrees F. Stir the chopped chocolate and marmalade together in a small bowl until combined.
Sprinkle a work surface and rolling pin with confectioner's sugar and roll out dough to 1/8-inch thick. Move dough frequently to prevent it from sticking and add more confectioner's sugar to the work surface and rolling pin as needed. With a 2-by-2 1/2-inch heart-shaped cookie cutter, cut 48 hearts, re-rolling dough as needed. Place half of the hearts at least 1 inch apart on two parchment-lined baking sheets.
Evenly spread about 1 level teaspoon of the marmalade/chocolate mixture over each heart, leaving a 1/4-inch border around the edges. Brush the outer edge of the hearts lightly with egg-white wash. Place another dough heart over the filling and gently press the edges to seal. Continue with remaining dough cut-outs. Lightly brush the surface of the cookies with egg-white wash and sprinkle generously with sanding sugar. Bake until set, about 12-14 minutes. Let cool on the pan slightly before transferring to a rack to cool completely.
Powdered sugar, for rolling out dough
Sift together flour, baking powder, and salt. Set aside. Place butter and sugar in large bowl of electric stand mixer and beat until light in color. Add egg and milk and beat to combine. Put mixer on low speed, gradually add flour, and beat until mixture pulls away from the side of the bowl. Divide the dough in half, wrap in waxed paper, and refrigerate for 2 hours.
Preheat oven to 375 degrees F.
Sprinkle surface where you will roll out dough with powdered sugar. Remove 1 wrapped pack of dough from refrigerator at a time, sprinkle rolling pin with powdered sugar, and roll out dough to 1/4-inch thick. Move the dough around and check underneath frequently to make sure it is not sticking. If dough has warmed during rolling, place cold cookie sheet on top for 10 minutes to chill. Cut into desired shape, place at least 1-inch apart on greased baking sheet, parchment, or silicone baking mat, and bake for 7 to 9 minutes or until cookies are just beginning to turn brown around the edges, rotating cookie sheet halfway through baking time. Let sit on baking sheet for 2 minutes after removal from oven and then move to complete cooling on wire rack. Serve as is or ice as desired. Store in airtight container for up to 1 week.