Heat the oven to 350°F (175°C). Grease and flour a 10-inch tube or Bundt pan thoroughly, paying close attention to the central tube and decorative ridges, or coat it well with baking spray.

Combine the brown sugar, flour, cinnamon, butter, chopped nuts, and optional salt in a bowl. Stir or rub the mixture together until it forms coarse, crumbly clusters, then set it aside.

Whisk the flour, baking powder, baking soda, and fine salt in a separate bowl. Breaking up any small lumps now helps the dry ingredients distribute evenly through the batter.

Beat the softened butter and granulated sugar for 3–4 minutes, until pale and fluffy. This creaming step adds air to the batter and supports a lighter cake texture.

Beat in the eggs one at a time, allowing each egg to blend in before adding the next. Mix in the vanilla extract, scraping the sides of the bowl if needed.

Add the dry ingredients in two or three additions, alternating with the sour cream. Mix only until the batter is combined, because extended mixing can develop gluten and make the cake dense.

Spread about half of the batter into the prepared pan and sprinkle roughly half of the streusel over it. Add the remaining batter, then finish with the remaining streusel; for a Bundt cake, keep the filling concentrated in the middle rather than letting it sit directly against the pan.

Bake for 40–50 minutes, until a toothpick inserted into the cake portion comes out clean or with a few moist crumbs. Begin checking near the 40-minute mark, since pan shape and oven performance can affect the final timing.

Cool the cake in the pan for 15–20 minutes. Invert it onto a rack, let it cool further before slicing, and serve plain or with an optional powdered-sugar glaze.