In a large mixing bowl, beat butter and sugar together until light and fluffy. This takes about 2-3 minutes on medium speed. On low, beat in the eggs and vanilla extract until smooth and well combined. Alternate adding dry ingredients and buttermilk to the wet.
Evenly spread half coffee cake batter into the prepared pan. Use ? to ½ of the streusel to top the first layer of cake. Repeat with a second layer of coffee cake and finish with the remaining cinnamon streusel.