Make the filling. Stir all the filling ingredients together in a medium bowl. Set aside.
Make the streusel. Mix the brown sugar, flour, and cinnamon in a bowl. Work in the butter until crumbly, then stir in the pecans.
Make the batter. Heat the oven to 325 F and grease the pan. Beat the butter and sugar with a mixer until light and fluffy. Add the eggs one at a time, beating well after each, then mix in the vanilla. In another bowl, sift together the flour, baking powder, and baking soda. Add the dry ingredients to the butter mixture in parts, alternating with the sour cream and mixing after each addition.
Layer it. Spread half the batter in the pan and sprinkle with half the streusel. Spread all the pumpkin filling over the streusel. Spoon the rest of the batter on top and gently spread it. Finish with the remaining streusel.
Bake about 50 minutes, until a toothpick inserted into the cake near the center comes out free of wet batter. Cool in the pan on a wire rack.
Add a glaze (optional). For a bakery-style finish, whisk 1 cup powdered sugar with 1 1/2 to 2 tablespoons milk until smooth. Drizzle it over the cooled cake.
Notes
The cake comes together faster if you make the filling first, then the streusel, then the batter.