- Make the shortbread crust.
- While the crust bakes and cools, make the cranberry curd: Put cranberries, sugar and orange juice and peel in a saucepan over medium heat. Simmer until cranberries have popped and softened, about 10 minutes (do not let the mixture cook down - you will want enough to cover the entire surface of the crust).
- For the most vibrant color, purée the cooked cranberry and orange mixture with an immersion blender or in a food processor or blender. Press the resultant puree through a fine-mesh sieve. (This should yield 1 - 1.5 cups of liquid if it wasn't cooked down too much.)
- Put eggs and egg yolks into a bowl and beat lightly. Slowly whisk a cup of warm cranberry liquid into the eggs to temper, then combine both and whisk together. Add corn starch and whisk well. Wipe out pot if necessary, return liquid to pot and cook over low heat until nearly bubbling and thickened, about 10 minutes. Whisk the butter into the warm liquid one cube at a time, making sure each cube combines completely before adding the next one.
- If using immediately, let cool to room temperature. If working ahead, cool to room temperature, cover with plastic wrap (press wrap against curd) and refrigerate. (Curd may be cooked up to 1 day ahead.)
- Pour cooled cranberry curd into the cooled prebaked tart shell and smooth top with a spatula. Bake at 350 degrees for 10 minutes to set curd. Cool on a rack. Store at room temperature for up to 2 days.