For the shortbread base:

1. Preheat oven to 350°F/180°C.
2. Butter or spray an 8-inch square pan and line with a wide strip of parchment paper. It will cover the bottom and two of the sides. 
3. In a large bowl mix soft butter with powdered sugar. It should be creamy.
4. Add flour and patiently incorporate it all until you have a soft dough. I use a spatula or a wooden spoon. 
5. Put this dough in the bottom of the prepared pan in small mounds. With your fingers pat until it covers the whole surface. 
6. Prick with a fork and bake for 15-20 minutes, until starting to dry and barely color. Reserve on a wire rack while making the filling.
7. Leave the oven on.

For the filling:

1. Put butter, honey and brown sugar in a medium saucepan. 
2. Cook on medium heat until it breaks a boil. It will start to bubble. 
3. Cook for 5 minutes stirring occasionally. It will foam and maybe you want to lower the heat a bit. 
4. Remove from heat and immediately add the walnuts, cream, salt, vanilla, and lemon.
5. 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. 
6. Remove from the oven and let stand 5-10 minutes on a wire rack. 
7. Carefully run a smooth-bladed knife around the edges so that they don't stick when it cools completely. 
8. Cut into squares and serve. Or refrigerate for 1 hour after it has cooled down completely and then serve. The squares will hold better.