In the bowl of a stand mixer fitted with the paddle attachment, cream the butter on low speed until it’s smooth and there are no lumps. Scrape down the bowl and paddle often.
Add the cream cheese and continue mixing until smooth, scraping often. You want to make sure none of the cream cheese or butter is sticking to the paddle, or it may end up creating lumps. Mix in the vanilla, lemon extract (if using), and lemon juice.
Slowly add the confectioners’ sugar and then the golden syrup (if using) and continue mixing until smooth.
If the frosting is thin and soft, you may want to chill it to give it more body and allow you to fill and decorate with more ease. Just cover the bowl and refrigerate until the frosting is firm, up to 24 hours, then return to the stand mixer fitted with the paddle attachment and mix on low speed until smooth.
Use immediately.