Combine the brown sugar, flour, cinnamon, and salt in a small bowl.
Add the melted butter, and stir until thoroughly combined. Set aside.
To make the cake:
Preheat oven to 350°F (175°C). Line a 9" x 5" loaf pan* with parchment paper so that it overhangs on the long sides. Grease the paper. (If you don't want to use parchment paper, simply grease the pan.)
Whisk together the flour, baking powder, baking soda, and salt. Set aside.
Using an electric mixer on medium speed, beat the butter and sugar until light and fluffy. Add the eggs, one at a time, mixing well after each addition. Mix in the vanilla.
Reduce mixer speed to low. Mix in the flour mixture in three portions, alternating with two portions of sour cream. Mix just until combined.
Transfer the batter to the prepared pan, and spread evenly.
Sprinkle the crumb topping over the top of the batter.
Bake 40 to 45 minutes, or until the cake has browned and a pick inserted into the center comes out clean.
Cool the cake in the pan on a wire rack for about 15 minutes. Then use the overhanging parchment paper to lift the cake out of the pan to cool completely on a wire rack.
Notes
*Make sure of your pan's dimensions, as there are two sizes of loaf pans sold as "standard." If you use the smaller size, then your cake will take longer to bake and may not bake thoroughly and evenly.
Store in an airtight container at room temperature up to 3 days.