This shortbread is so rich and flavorful that you may not wish to add icing; in that case, substitute granulated sugar for flour when rolling out the dough.
Sift together dry ingredients and process in a food processor until well mixed.
Add the butter pieces and process until mixture becomes a fine crumble (it should resemble cornmeal).
Melt chocolate in a double boiler over medium-low heat. In a small bowl, combine yolks, heavy cream, and vanilla.
When chocolate is melted, remove from double boiler and cool until just warm to the touch. Gently heat the yolk mixture in the double boiler for 1 minute, until yolks are the same temperature as the chocolate. Do not overheat, or they will curdle.
Add yolk mixture to melted chocolate and mix well. Pour into food processor along with flour mixture, and mix until dough comes together, 3 or 4 pulses.
Shape dough into 2 flattened rounds. Wrap and chill for at least 1 hour.
Heat oven to 325 degrees, and line baking sheets with parchment.
On a well-floured surface, roll dough 1/8 inch thick. Dip cutters in flour and cut into desired shapes. Brush off any excess flour. Carefully transfer cookies to sheets with a metal spatula, leaving a 1-inch space between them.
Bake for 10 to 14 minutes; do not overbake. Cool completely on racks, then decorate with Lemon Royal Icing.