Place the chocolate in a small bowl, then add the boiling water. Let stand for about a minute, then whisk the mixture until smooth. Sift the flour, cocoa powder, confectioners’ sugar and salt together into a large bowl. In a measuring cup, whisk together the eggs and vanilla until combined.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and both sugars on medium speed until light and fluffy, about 3 minutes. Scrape down the sides of the bowl, then, with the mixer on low, add the egg/vanilla mixture and beat just until incorporated (the mixture might look a little curdled, it’ll come together don’t worry).
Pour the batter into the prepared pan, spreading it evenly. Bake for about 45 minutes, or until the edges of the cake start to pull away from the pan and the top springs back when lightly pressed (if you halve the recipe and bake in a 6-cup bundt, the baking time will be less, mine was about 32 minutes).
Transfer the pan to a wire rack and let the cake cool in the pan for about 1 1/2 hours before turning it out onto the rack. Let the cake cool completely on the rack before glazing, about 2 more hours.