Preheat oven to 350°F (regular bake setting/not fan assisted). Grease a 9-inch square baking pan and set onto a baking sheet. Sprinkle chopped walnuts evenly over the bottom of the pan. Set aside.
In a large bowl with an electric mixer or in the bowl of a stand mixer fitted with a paddle attachment, beat butter with brown sugar until light and fluffy. Add egg and vanilla and mix until combined.
In a medium bowl, whisk together the flour, baking powder, and salt.
Add flour mixture to batter in 3 additions, alternating with the milk. Mix just until combined. Spoon into prepared pan and spread into an even layer.
Make sauce by combining sauce ingredients in a medium saucepan. Heat over high heat until mixture comes to a boil, then reduce heat to medium and boil for 2 minutes. Carefully and slowly pour sauce mixture over batter in pan. Do not stir.
Bake with the pan on a baking sheet in preheated oven for 30-40 minutes, or until cake is set (pulling away from sides of the pan) and golden.
Serve warm with a drizzle of cream or a scoop of ice cream.