1. Preheat the oven to 350° F (177° C).
2. Line the bottom of a 9 inch cake pan with parchment paper. Grease the bottom and sides of the pan with butter or nonstick spray, set aside.

Make the cranberry layer:

1. Melt the 4 tablespoons of butter in a large skillet set over medium heat. Once the butter is melted, add the 1 cup packed brown sugar, 1/3 cup granulated sugar, 1/4 cup water and pinch of salt. Stir to combine. Once the sugar begins to dissolve, add the cranberries. 
2. Stir the mixture frequently until the cranberries start to pop and soften, about 3 - 4 minutes. (Be careful, there may be a bit of splatter when the berries start to pop!) To avoid burning the sugar, turn the heat down to medium low if necessary.
3. Remove the pan from the heat and allow the sugar syrup to stop bubbling. Then, remove the cranberries from the pan with a slotted spoon and place them in the prepared pan in an even layer. 
4. Place the pan back on medium heat, and allow the sugar mixture to continue reducing in the pan for another 3 - 4 minutes, or until syrupy. Stir frequently and watch the pan to avoid burning the sugar, turn the heat down if necessary. Once the syrup has reduced, pour on top of the fruit in an even layer.

Make the cake batter:

1. Cream together the 8 tablespoons softened butter with the 1/4 cup packed brown sugar and 1/2 cup granulated sugar in a large bowl. (See notes.) Whisk in the eggs, one at a time.
2. Whisk together the flour, salt and baking powder in a separate bowl. Whisk the dry ingredients into the wet ingredients gradually, alternating with the milk. Whisk in the vanilla. Continue whisking just until the batter is smooth, but do not overmix.
3. Pour batter on top of the cranberries, spreading gently to the edges of the pan. 
4. Bake for 45 - 50 minutes, or until a toothpick inserted into the center comes out clean. Wait 10 minutes before turning over onto a cake stand or plate.