1. Toss the angelica with a bit of the flour to separate the pieces and set aside.
2. Butter a 10-inch (27cm) bundt pan and dust the inside with powdered sugar, then tap out any excess.
3. Preheat the oven to 300ºF (150ºC.)
4. In the bowl of a stand mixer, stir together the egg yolks with the sugar. Whip the yolks on high speed until thick and lightened in color. With the mixer running on high speed, dribble in the very hot water, a few teaspoons at a time, then add the vanilla. Continue whipping until the mixture has rethickened and resembles soft pudding, holding its shape when you lift the whip.
5. In a separate bowl, whip the egg whites with the salt and cream of tartar until they hold stiff peaks.
6. Put the cake flour in a mesh strainer or sifter, and sprinkle the flour over the yolks, folding it into the yolks gradually as you sift.
(If you’re not so dexterous, scrape the whipped yolks into a larger bowl, set the bowl in the center a damp kitchen towel formed into a circle on the countertop, and use one hand to sift in the flour and the other to fold it in.)
7. Stir in the candied angelica then fold in one-third of the beaten egg whites until fully incorporated. Then fold in the remaining egg whites.
8. Scrape the batter into the prepared pan and bake for 1 hour. It’s done when a toothpick inserted into the center comes out clean, with perhaps just a few crumbs attached.
Remove from oven and immediately turn the cake out onto a wire cooling rack. Let cool completely before slicing.