Preheat your oven to 350°F / 175°C degrees, racks in top and bottom third. Line a couple baking sheets with parchment paper.
Finely chop the chocolate bar into 1/8-inch pieces, more like shavings really. Try to avoid big lumps and chunks, which make flattening out the cookie dough later more difficult.
In a mixing bowl, whisk together the flour, baking powder, baking soda, salt, oats, walnuts, and shaved chocolate. Set aside.
Using a mixer (or by hand) beat the butter until fluffy. Beat in the sugar and mix until it is also light and fluffy. Scrape down the sides of the bowl once or twice. Beat in the molasses, then the egg, mixing until both are well incorporated, scrape down the sides of the bowl once or twice. Beat in the vanilla. Add the flour mix and stir by hand until the ingredients barely come together into a uniform dough.
I like these cookies tiny, barely bite-sized, so scoop out the dough in exact, level teaspoons. Then tear those pieces of dough in two before rolling each 1/2 teaspoon of dough into a ball shape. Place two inches apart on your prepared baking sheets. Gently flatten each dough ball into a thin, round patty with two fingers and then sprinkle the top of each cookie with a pinch of large-grain sugar.
Bake for 7 minutes or until cookies are golden and fragrant. Remove from oven, and cool on a wire rack.