In a large pot, combine whole milk, maple syrup, and sugar. Use a wooden spoon or spatula to stir the mixture gently the entire time.* Heat on medium-low heat until the mixture reaches 180°F. Meanwhile, whisk the egg yolks in a small bowl until light and pale yellow in color. Temper the yolks by adding a small amount (1/4 – ½ a cup) of the warm milk mixture into the egg bowl, stirring while you incorporate. Pour the warmed egg yolks into the milk pot, again stirring while you incorporate.
Continue to stir the mixture slowly on low heat for about 5 minutes. Your custard will be ready when the mixture begins to thicken and coats the back of a spoon (and reaches between 160-170°F). Don’t let the mixture cook for much longer, or the eggs will cook and the custard will curdle. Remove from heat and stir in heavy cream, vanilla and butter extracts, and salt. Pour into a freezer-safe container and allow to chill in the fridge for at least 12 hours (overnight is perfect!)
About an hour before churning your ice cream, make your blueberry swirl.In a small saucepan, heat the blueberries, sugar, and lemon juice on medium heat until the blueberries just slightly begin to release their juices. Remove from heat and allow to chill for at least 1 hour.
Once chilled, pour ice cream into your ice cream maker and churn according to manufacturer’s instructions. Once the ice cream is done churning, scoop half of it out into a container. Pour half of your chilled blueberries over the ice cream. Repeat this process with the second half of your ice cream and blueberries.
Use a spatula to gently swirl the blueberries into the ice cream. Don’t swirl too much, or the entire thing will become blue! Cover and allow to freeze for another 12 hours. Then serve!
*Be very careful not to whisk your mixture or stir to heavily, or it will foam and separate