Preheat oven to 350°F. Spray a tube pan with baking spray or butter and flour. Set aside.
Cake Batter: In a large bowl using a whisk, sift together flour, baking powder, baking soda and salt. Once combined, add sugar. Whisk until combined.
In a separate medium bowl whisk together buttermilk, butter, vanilla and eggs.
Add wet ingredients to dry ingredients. Mix by hand until fully moistened and lumps are dissolved.
Streusel: In a medium size bowl, mix together brown sugar, flour and cinnamon. Cut in butter using a pastry blender or rub with your hands until it resembles peas. Add 1 cup chopped pecans or walnuts. Mix well.
Assemble: Pour half of the cake batter into the tube pan. Sprinkle 1/2 streusel over batter. Top with remaining cake batter ending with streusel.
Bake for 55-60 minutes. or until a toothpick inserted into the cake portions shows moist crumbs. Check at 30 minutes and lay foil on top to prevent over browning, if needed.
Cool in the pan on a baking rack for 30 minutes. Remove from the pan and cool completely.
Glaze: Mix together powdered sugar and heavy cream. Drizzle over cooled cake.
Store in an airtight container at room temperature or chilled.