Preheat oven to 350°F (175°C)
Grease and flour 10-inch bundt pan
Toss apples with brown sugar & cinnamon
Make streusel by cutting butter into dry ingredients
Whisk flour, baking powder, baking soda, cinnamon, and salt
Cream butter and sugar until fluffy
Add eggs one at a time, mixing well
Mix in vanilla extract
Alternate adding dry ingredients and buttermilk
Pour half the batter into pan
Layer half of apples and streusel
Add remaining batter, apples, and streusel
Bake for 50-60 minutes
Cool in pan for 15 minutes before inverting