Make the cake: Preheat the oven to 325 degrees F. Butter a 10- to 12-cup Bundt pan, making sure to get in all the crevices; dust with flour and tap out the excess. Whisk the flour, baking powder, baking soda and salt in a medium bowl; set aside.
Beat the butter and granulated sugar in a large bowl with a mixer on medium-high speed until light and fluffy, about 5 minutes. Beat in the eggs one at a time, then beat in the sour cream and vanilla. (The batter may look curdled.) Reduce the mixer speed to medium low and add the flour mixture in three batches, alternating with the buttermilk; beat until combined.
Pour the batter into the prepared pan and smooth the top. Bake until the cake is golden and a toothpick inserted into the center comes out clean, about 55 minutes. Transfer to a rack and let cool 15 minutes in the pan; loosen the edge of the cake with a knife and invert onto the rack to cool completely.
Make the glaze: Whisk the confectioners' sugar, lemon juice, vanilla and 1 tablespoon water in a bowl; add a splash more water if the glaze is too thick. Spoon over the cake, letting it drip down the sides. Decorate with nonpareils. Let stand until set, 5 to 10 minutes. Put rock candy sticks in the center of the cake.