Preheat oven to 375 degrees Fahrenheit. Line or grease the cups of a muffin tray.

Prepare the jam. In a small pot, stir together blueberries and sugar over medium heat, mashing the blueberries down as you stir. After about five minutes, mixture should be a thick mush/jam. Remove pot from heat and let cool.

Using an electric mixer, cream together butter, oil, and sugar for a few minutes until mixture is light and fluffy. Add eggs, vanilla extract, and lemon zest and cream mixture for a few more minutes, until fluffy and smooth again.

In a small bowl, stir together flour, baking powder, and salt. In another small bowl (or in a measuring cup), combine sour cream and milk.

With machine running on low, alternate adding in the flour mixture, then the sour cream mixture, starting and ending with the flour mixture. Don’t overmix! Stir in blueberries gently by hand.

Using a large cookie scoop, drop batter evenly into prepared muffin cups. Drop a teaspoon of the blueberry jam on top of each muffin, and then swirl together the jam and muffin batter with a knife or toothpick.

Bake for about 20 to 28 minutes (depending on muffin size and depth), or until muffin springs back when touched and a toothpick inserted into the center of one comes out clean or with a few moist crumbs. Don’t overbake.