Toast the walnuts; Preheat oven to 160°C. Arrange walnuts on a baking sheet in a single layer. Bake for about 10 minutes, checking frequently.
Let cool and blitz the nuts in the food processor for about 10 seconds or so depending on how efficient your food processor is.
Stir together ground walnuts with brown sugar in a separate small mixing bowl. Set aside.
Grease an 18 cm cake pan and line the base with parchment paper. Preheat the oven to 180°C.
Sift together flour, baking powder, and salt 3 times.
Have all the ingredients at room temperature before starting.
In the bowl of the food processor, pulse the butter until creamy depending on how efficient your food processor is.Scrape down the sides of the bowl as needed.
Add the sugar in 3 additions and pulse together until well combined and light in color. Scrape down the sides.
Whisk the eggs, honey and rum together in a small bowl and add, one tablespoon at a time, pulsing well after each addition.To stop the batter from curdling, add one spoonful of the flour mixture if required.
Add the remaining flour mixture, and process with 3 or 4 quick on/off turns, just until flour disappears. Scrape down the sides as necessary.
Add the ground nut mixture, pulsing until just combined.
Transfer the batter to the prepared pan and spread evenly with a spatula.
Bake in preheated oven at 180°C for 20 minutes and then at 170°C for 20 minutes or until a toothpick inserted into the center comes out clean.
Allow rest in the pan for 5 minutes before transferring to a wire rack to cool completely.
Let cool on a wire rack. Serve & enjoy!