Preheat the oven to 350 degrees F. Make sure the rack is in the middle of the oven. Prepare a 9″ springform pan.
Whisk the flour, baking powder, baking soda, pumpkin spice, and salt together in a medium-sized bowl until evenly combined.
Using a hand mixer, beat the butter and brown sugar in a separate bowl until completely combined. Slowly beat in the eggs, one at a time. Then add in the pumpkin puree and sour cream. Beat slowly until the ingredients are just combined. Pour in the flour and fold in with a wooden spoon or rubber spatula until the batter is even, smooth, and thick. Set aside.
In a separate small bowl, mix together the flour, brown sugar, granulated sugar, cinnamon, and salt for the streusel. Add the pieces of butter, and use 2 knives to cut the butter into the flour mixture until coarse crumbs (about the size of peas) form. Using 2 knives or a pastry cutter, cut the butter into the flour mixture until coarse crumbs form. Set aside.
Pour about half of the cake batter into your springform pan. Sprinkle with about a quarter of the streusel. Then pour over the remaining cake batter and smooth it out slightly with a wooden spoon or rubber spatula. Top it with the remaining streusel.
Bake the cake for 50 minutes, or until an inserted toothpick comes out clean. Cool in a pan on a cooling rack for 15 mins, then remove from pan and finish cooling on the rack.
As the cake cools, make the Maple Caramel.
Melt the butter in a medium-sized saucepan over medium-high heat. Add the sugar and the salt.
Cook and stir the mixture until the sugar completely dissolves. Then turn the heat down to medium and boil for 2 more minutes. Add the maple syrup and the mixture once again come to a boil. Stir frequently until sauce has thickened and coats a spoon, about 3 minutes longer. Remove from heat and cool, and then spoon over slices of cake.