Place the chopped dates in a small bowl and pour the Armagnac over them. Let them soak for 20 minutes while you prepare the rest of the batter.
Heat the oven to 350°F and line a small loaf pan with parchment paper.
Whisk together the flour, baking soda, and salt in a medium bowl.
In a separate bowl, beat the brown sugar and butter until light. Add the egg and mix until smooth.
Stir the soaked dates and Armagnac into the wet mixture. Fold in the dry ingredients until just combined, then add the walnuts.
Spoon the batter into the prepared pan and smooth the top. Bake for 35 to 40 minutes, until a toothpick comes out clean. Cool completely in the pan.
Beat the mascarpone with the powdered sugar until creamy. Spread over the cooled loaf before slicing.