Whisk all-purpose flour, rye flour, cinnamon (or ginger) and salt in a medium bowl.

Fit another medium bowl into a pan of gently simmering water—don't let the water touch the bottom of the bowl. Put butter in the bowl, then put dark chocolate on top. Heat, stirring occasionally, until the butter and chocolate are melted. Remove the pan from the heat and whisk in granulated sugar. Add eggs, one at a time, and beat energetically until the mixture is thick and smooth, about 1 minute. Beat in vanilla. Stir in the flour mixture with a flexible spatula until the dry ingredients disappear into the dough. Cover and refrigerate for at least 2 hours or up to 1 day.

Preheat oven to 350°F. Line 2 baking sheets with parchment paper or silicone baking mats. Put turbinado sugar in a small bowl.

Using a scant tablespoon per cookie, roll the dough into balls between your palms. Roll the balls around in the sugar to coat. Place the balls on the prepared baking sheets (13-15 per pan), giving them 1 1/2 inches or so of room to spread. Make an indentation in the center of each cookie with a wine cork or your thumb.

Bake 1 pan at a time until the cookies feel almost firm, 10 to 12 minutes. Transfer the pan to a wire rack. The indentations will have puffed in the oven, so gently press them down again.

If filling the cookies with ruby chocolate, melt it, either in a microwave or in a bowl over simmering water. Using a small spoon, fill each indentation to the brim with chocolate. Pop the cookies into the fridge for about 10 minutes to set the chocolate. If filling with jam, stir in a splash of warm water to thin it and then fill the cookies while the jam is still liquid. If the cookies are still warm, let them cool to room temperature before serving.