To make blueberry syrup, combine blueberries, ½ cup sugar, limoncello, lemon zest and water in a small saucepan. Mash the berries with a potato masher and bring the mixture to a boil. Cook on medium high heat until it has thickened and reduced by ½, around 6-7 minutes.
Pour syrup into a medium heavy saucepan, through a fine mesh sieve, pushing hard on the solids to extract all the liquid. Add lemon juice, corn syrup and 1 cup sugar. Stir occasionally with a wooden spoon until the mixture begins to boil. After it begins to boil, turn the heat down to medium, cover with a lid and let it continue to boil for two minutes (watching that it doesn’t boil over).
Uncover the saucepan and drop in butter. Continue cooking until a candy thermometer reads 235-245 degrees. Remove the saucepan from the stove and add salt and baking soda. Combine well. Pour over the popcorn in a large bowl and stir until well coated.
Spread the popcorn out onto a cookie sheet covered with parchment paper. Now, at this point, you can let it cool and eat it! However, for a crisper texture, you can place the popcorn (on the cookie sheet) in a 200?F oven for an hour, stirring every 15 minutes.
