Soak poppy seeds overnight in buttermilk.
Pre-heat oven 350 degrees.
Grease bundt pan with butter or Crisco then sprinkle 1/3 of the sugar/cinnamon mixture on the inside of the pan until coated.
Cream butter. Add one cup sugar gradually. Beat until fluffy then add eggs, one at a time, beating well after each addition. Add vanilla and beat until mixed in.
In a separate bowl, sift flour, salt, and baking soda together.
Alternate between mixing a portion of the butter mixture and a portion of the flour mixture into the poppy seed/milk mixture. Blend well.
Pour 1/3 of the batter into the bundt pan then sprinkle with 1/2 of the remaining cinnamon/sugar mixture. Repeat with the next 1/3 of the batter. Finish by pouring the final 1/3 of the batter in the pan.
Bake for about 50 minutes.