Preheat the oven to 350 degrees F
Soak the poppyseed in the buttermilk, adding the vanilla and string the mixture together.
Combine the half cup sugar and 1 tsp cinnamon for the sugar mixture into a small bowl. Set aside.
Separate the egg whites from the yolks, putting the whites into a large bowl and the yolks into a small dish. Beat the whites with a mixer until very stiff and set aside.
Cream together 1.5 cups sugar and the butter in a large bowl or stand mixer. Once combined, add the egg yolks and mix everything together.
Combine the flour and the baking soda in a medium bowl (sifting together is best, but mixing with a wooden spoon will work as well).
Alternately add the flour mixture and buttermilk mixture to the butter mixture and stir until combined.
Fold in the egg whites to the batter with a spatula, until whites are well combined.
Coat the angel food cake pan with oil, butter, or other fat on the inside to prevent sticking.
Scoop a third of the batter into the pan with a spoon or spatula, spreading in an even layer to cover the bottom of the pan.
Spoon half of the sugar mixture into the pan on top of the batter in an even layer.
Scoop another third of the batter evenly on top of the sugar mixture.
Top the batter with the remaining sugar mixture, in an even layer.
Place the remaining batter evenly on top of the sugar mixture.
Place the pan in the oven and bake for 50-60 minutes, until cake is browned on top and a toothpick inserted into the cake comes out dry.