Streusel: Preheat oven to 350 F. Now Grease a 9-inch square baking pan.
Mix the brown sugar, cinnamon, and flour until well combined. Add the butter and stir in the pecans. Mix in the pecans then set aside. To make the cake, mix the flour, baking powder, and salt with the baking soda, allspice, and baking powder. After this, In a large separate bowl mix the butter, sugar, and sour milk. Now mix eggs, and vanilla, and whisk them together.
After this, Mix this with the flour mixture until it is well combined. Place half the batter in the prepared pan. Sprinkle half the streusel over the top. To swirl the streusel a little, use a paring knife or a skewer.
Now Spread the batter on top of the cake, then add the rest of the streusel. After this, Give the cake a final swirl with the remaining streusel.
Next, Bake the cake for 45-55 minutes or until a toothpick inserted into the middle of the cake comes out clean. When slicing, cool the cake in its pan to room temperature.
The cake can be kept in an airtight container for up to three days.