Put 6 tablespoons unsalted butter, ½ cup light brown sugar and ½ cup runny honey in a medium saucepan.
Cook on medium heat until it boils, stirring occasionally. It will foam, so you may want to lower the heat a bit.
Remove from heat and immediately add 1 cup walnuts, ¼ cup heavy cream, pinch salt, 1 teaspoon vanilla extract, and ½ teaspoon lemon zest.
Pour into the pre-baked crust and return to the oven for 20 minutes. It will be bubbling and still look wet and somewhat runny, but not as much as before baking. The filling hardens a lot when it cools down, and if you bake it too much it will become so hard it will be almost impossible to eat.
Remove from the oven and let stand for 5-10 minutes on a wire rack.
Carefully run a smooth-bladed knife around the edges so that they don't stick when you remove them.
Let the pan cool completely on a wire rack and refrigerate for 30 minutes.
Lift the paper to remove the whole block from the pan. Cut into squares and serve. The squares will hold better if they're chilled first.