Special equipment: A 10- to 15-cup bundt pan
For the cake: Preheat the oven to 350 degrees F. Butter and flour a 10- to 15-cup bundt pan.
In a large bowl, whisk together flour, baking powder, salt, and baking soda and set aside. In a large bowl, beat the butter and granulated sugar with an electric mixer on medium until light and fluffy, 3 to 5 minutes. Add the orange zest. Then beat in the eggs and egg yolks 1 at a time. Beat in the poppy seeds and vanilla extract. Alternate adding the flour mixture and the buttermilk, beginning and ending with the flour mixture.
Transfer the batter to the prepared pan and smooth the top. Bake until a toothpick inserted into the center comes out with moist crumbs attached, 45 to 50 minutes. Transfer to a rack to cool for 15 minutes, then flip the cake out and let it cool completely.
For the glaze: In a medium bowl, whisk together the confectioners' sugar and orange zest and juice. Drizzle the cake with the glaze to serve.
Creaming the butter--the mixing of the butter and sugar--is one of the most important steps in cake-making. A recipe will often instruct bakers to cream the butter and sugar for somewhere between 3 and 5 minutes. That may seem like a long time, but it is in fact important. Creaming the butter and sugar together actually aerates the batter, leading to a light and fluffy cake. Just be sure to start with room temperature butter and to scrape down the bowl occasionally.