Begin by making the crumble. Preheat oven to 350℉. Line a rimmed baking sheet with parchment paper. Combine the melted butter, brown sugar, salt, and vanilla in medium bowl. Whisk to combine. Stir in the flour until mixture forms a crumble. Pour onto baking sheet in an even layer and bake for 15 minutes or until golden, stirring once during baking. Set aside to cool.

In a medium saucepan, heat blueberries with ¼ cup of the sugar, stirring, until the blueberries begin to break down and become juicy, about 8 to 10 minutes. Set aside to let cool, and then refrigerate to chill them.

In a large saucepan, combine the remaining sugar, lemon juice, heavy cream, milk, and vanilla. Heat over medium-low and cook, stirring constantly, until the mixture just begins to simmer. Do not let the mixture boil.

In a bowl, whisk together egg yolks until smooth. Gradually pour half of the hot milk and cream mixture into the egg yolks, whisking constantly. Gradually pour the egg yolk mixture back into the hot mixture in the saucepan while whisking constantly. Cook, stirring, until the mixture thickens and coats the back of a spoon.

Strain the mixture with a fine mesh strainer into a bowl. Refrigerate until chilled or chill in an ice bath if you're short on time. Fold in the cold blueberry mixture.

Churn according to your ice cream machine manufacturer's instructions. Once ice cream has churned, fold in the pistachios and ¾ of the crumble. Pour into a loaf pan or vessel for freezing and freeze for 4-6 hours. Top with the extra crumble for serving!