Preheat the oven to 325 degrees Fahrenheit. Line a large baking sheet with parchment paper. In a large bowl add the flour, baking powder, cardamom, brown sugar, and orange zest - mix it together well, using your hands to break up any brown sugar lumps.
Stir in the beaten eggs with a large wooden spoon. Using your hands, bring the dough together until no floury bits remain. Work in the dried currants and walnuts until everything is evenly distributed.
Divide the dough into two pieces, and lightly flour a non-stick surface. Shape each piece of dough into a 10 inch log. Carefully set the dough on the parchment lined baking sheet, spaced far apart. Set on the middle rack and bake for 30 minutes.
Remove the logs from the oven and allow them to cool slightly. Transfer to a cutting board and using a serrated knife, cut into slices about a ½ inch thick on a diagonal. Lie the slices flat on the baking sheet and dust each one with turbinado sugar. Reduce the oven temperature to 250 degrees Fahrenheit and return the slices back to the oven.
Bake for another 20 minutes, turn them over dust with more turbinado, and then bake again for another 20 minutes. The thimblets should be dry and crunchy all the way through. Serve with a freshly pulled shot of espresso. Stored in an airtight container, thimblets will stay good for several weeks.
