Make the blueberry sauce. Add the ingredients for the sauce to a small saucepan and heat to medium-high. Lightly mash and continue to cook until the liquid forms a thick syrup. Set aside to cool.
Start the cupcakes while the sauce is cooling.
Preheat the oven to 350F. Line the cupcake tin with liners. Set aside.
Mix together the flour, baking powder, and salt together in a medium bowl and set aside.
Add the butter, sugar, and lemon zest in a mixer and beat until light and fluffy - about 4 - 5 minutes on medium speed.
Beat in the eggs one at a time. Make sure you beat well after you add each egg. Beat in the vanilla.
Mix together the buttermilk and lemon juice in a small bowl.
Alternate between adding the flour (in three additions) and the buttermilk/lemon mixture (in two additions). Stir until just combined.
Fill the cupcake liners with the batter so that it is about ⅔ full. Add 1 ½ teaspoons of the blueberry sauce to the batter and use a toothpick to swirl.
Bake for 15 - 18 minutes. Remove from the oven and allow to cool to room temperature.
Make the frosting. Beat the butter and cream cheese until it is smooth and creamy. Add the lemon zest and salt, then the powdered sugar one cup at a time, beating well between each addition.
Separate about ⅓ of the frosting and place in a small bowl. Mix in the blueberry sauce.
Fit a piping bag with your desired piping tip. Begin adding both frostings, using mostly the regular frosting with spoonfuls of the blueberry frosting added in sporadically.
Pipe the frosting over the cupcakes and garnish as desired!
Notes
Store them at room temperature for up to two days.