Slice the vanilla bean in half lengthwise, then use the dull side of the knife to scrape out the seeds. Set aside. (Skip this step if using vanilla extract).
Place the mascarpone cheese in a stand mixer fitted with the paddle attachment. Turn the mixer on medium-low to loosen the mascarpone, then turn up the speed to medium. Slowly add in the vanilla seeds, the heavy cream, and the sugar, until evenly combined. This can also be done in a large bowl with a whisk. (Note: mascarpone cheese and heavy cream can separate if they're not combined at the same temperature. This is easy to fix; see notes).
Transfer the mascarpone mixture into a piping bag or a large plastic storage bag with the corner snipped off. Pipe the mixture into the strawberries. If not serving immediately, place in the refrigerator to chill.
Before serving, top with chopped candied nuts, if using.
Notes
If you want to serve the strawberries so that they stand upright as opposed to laying on their sides, slice off the very bottom tips before piping in the filling.If the mascarpone cheese and heavy cream happen to separate, it just means their temperatures weren't harmonious enough to combine evenly. The quickest way to fix this is to scoop 1-2 tablespoons into a small bowl, microwave for 10 seconds, and whisk back into the mixture. If necessary, repeat this process again until the ingredients come together. You can also whisk the entire mascarpone mixture briefly over a double boiler. Once it comes together, chill in the refrigerator to firm the cheese back up.Candied nuts: I used the ones from this recipe.