Preheat the oven to 175°C, grease the pan, and line its base with parchment. Let the butter soften while you prepare the batter and walnuts for baking at the start.
Beat butter and sugar until pale, then add eggs one at a time. Mix thoroughly after each addition, then fold in walnuts for a smooth, aromatic batter. Mix until smooth.
Whisk flour, baking powder, and salt separately. Add them gradually with milk, folding just until combined, then keep the walnuts evenly distributed throughout the batter. Mix until smooth and glossy.
Fold the walnuts through the batter, keeping delicate pieces intact and distributing every flavor evenly from the center toward the edges of the prepared pan inside it. Mix until smooth.
Pour the batter into the prepared pan, smooth the surface, and bake until the center springs back lightly. Watch the walnuts for even browning during the final minutes of cooking.
Cool the cake completely on a rack before adding the prepared topping, because warmth can soften the layers and make the walnuts shift during assembly. Cool until set before serving.
Spread the prepared topping evenly, add the reserved walnuts or garnish, and chill the finished dessert until firm enough to slice cleanly without dragging across layers. Shape the edges evenly.
Cut portions with a warm sharp knife, wiping the blade between slices. Serve slightly chilled so the crumb, cream, and walnuts retain distinct textures when plated for guests to enjoy.
