Grease a loaf pan with butter and add a piece of parchment paper along the bottom and up two of the sides for easy removal. Grease the parchment paper as well, just to be sure nothing sticks.
Add the butter to the bowl of your stand mixer fitted with the paddle attachment and mix on high speed until the butter becomes pale in colour (about 4-5 minutes).
Add the sugar and beat on high speed until the mixture is extra light and fluffy (about 7-10 minutes, stopping to scrape down the sides of the bowl every few minutes). The mixture should turn a very pale yellow colour and be quite fluffy after 7 minutes.
Beat in the eggs one at a time and continue beating on high speed for about 5-6 minutes after they've all been added to incorporate even more air into the batter.
Add the sour cream and the vanilla to the bowl and beat for another minute or so, scraping down the sides of the bowl to make sure everything is incorporated.
Add the orange zest and beat on high speed until combined.
Add the flour and salt to the mixing bowl and fold it into the wet ingredients by hand using a rubber spatula. Be sure to do this carefully so you don't deflate the batter as the flour is incorporated. Folding requires that you scrape the bottom of the bowl and use an "up and over" motion. Do not stir, as this will break the air bubbles you've created by beating the ingredients up until this point. This cake contains no leavening agent, so the bubbles in the batter are what will keep it from sinking in the middle as it bakes.
Carefully scrape the batter into the loaf pan you've prepared.
Bake for 65-75 minutes at 325 degrees Fahrenheit. After 65 minutes, test the cake by gently inserting a wooden skewer in the centre. If it comes out clean, it's done! If not, return the cake to the oven for another 5 minutes and test again.
When the cake is done, remove it from the oven and let it rest in the pan for about 10 minutes.
Remove it from the loaf pan using the parchment paper to a wire rack to cool completely before glazing.
For the glaze:
Whisk together the powdered sugar and orange juice and zest until the desired consistency is reached. It should be thick, but easy to drizzle. If necessary, add a bit more powdered sugar or orange juice (1/2 teaspoon at a time) until the desired consistency is reached.
Drizzle the glaze over the cake once it has cooled completely and decorate with a few orange slices.
Slice and serve .
To make this cake a day ahead, prepare the cake and store in an airtight container after cooling completely to room temperature. Glaze just before serving.