Preheat oven to 350℉. Spread 1/4 cup of pine nuts evenly onto small baking tray, bake for approximately 10-12 minutes until golden. Set aside and allow to cool. In a food processor bowl combine basil, 1/4 cup of untoasted pine nuts, cheese, garlic, lemon juice and salt. Pulse until the mixture is broken down and finely chopped. With processor running, gradually add oil and process until smooth, approximately 10 seconds. Set aside. Makes about a 1/2 cup.
Bring a large pot of salted water to a boil, add potatoes and cook until very tender. Drain potatoes and return pot to low heat and allow any residual moisture to evaporate, 1-2 minutes. Mash potatoes or press through a ricer. Add butter, and mash just to combine. Mash in salt, pepper and enough milk to make creamy. Top potatoes with 3/4 of the pesto and fold into potatoes with a flat rubber spatula. Drizzle remaining pesto on top and garnish with toasted pine nuts. Serve immediately.
The basil pesto can easily be prepared while the potatoes are cooking.