Have all ingredients at room temperature if possible. Preheat an oven to 350F. Generously butter a 10-cup bundt pan and then lightly dust it with flour. Tap out the excess flour over your kitchen sink.
Sift together the dry ingredients: flour, baking powder, baking soda and salt.
Zest the lemon over a small bowl with the sugar. Rub the zest into sugar with your fingertips.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and the coconut oil until smooth. Add the lemon sugar. Beat for 4 minutes on medium high until fluffy.
Add the eggs, one at a time, beating for about 30 seconds after each one.Scrape down sides of the bowl and the paddle. Add lemon juice, vanilla and preserved lemon (if using) and mix well. Don't worry if it looks curdled.
Remove the paddle and fold in the dry ingredients with a spatula, one cup at a time, alternating with the buttermilk like this: 1 cup flour, 1/2 cup buttermilk, 1 cup flour, the remainder of the buttermilk, the remainder of the flour. Use a very light hand, and fold only until the streaks of flour have disappeared.
Scrape the batter into the prepared bundt pan and smooth the top. Place on the centre rack in the oven and bake until a toothpick inserted into the thickest part comes out clean; about 60-65 minutes.
Cool the bundt for a full 10 minutes in the pan set on a wire rack. Then invert the bundt pan onto the rack, tap gently on the bottom until the cake releases. Cool completely before glazing.
In a small bowl, stir together the powdered sugar and 3 tablespoons of lemon juice. It should be quite thick. Place the cake on a stand or serving platter and drizzle the glaze over the top. If the glaze is too thick, add a little more lemon juice, a few drops at a time.
Top cake with candied lemon slices. Serve at room temperature.
I've tried this cake with sour cream instead of buttermilk and with a combination of plain yogurt and buttermilk. The point is, use what you have on hand. This is what I tend to do, because we must have cake!This cake is best enjoyed on the same day it is baked, but it will keep, stored in an airtight container at room temperature for up to 3 days.