Prep: Heat oven to 350°F (175°C). Line a 12-cup muffin pan with paper liners.
Whisk dry ingredients: In a bowl, whisk flour, baking powder, baking soda, and salt.
Cream butter and sugar: In a large bowl, beat butter and sugar until light and fluffy, 2–3 minutes.
Add eggs and flavors: Beat in eggs one at a time.
Mix in lemon zest, lemon juice, and vanilla.
Combine with dairy: Mix sour cream and milk in a small cup. Add dry ingredients to the butter mixture in two parts, alternating with the sour cream mixture. Mix just until smooth.
Do not overmix.
Fill and bake: Divide batter evenly among liners (about 2/3 full). Bake 16–19 minutes, until tops spring back and a toothpick comes out clean.
Cool: Let cupcakes cool in the pan 5 minutes, then transfer to a rack to cool completely.
Make the frosting: Beat butter until creamy. Add 2 cups powdered sugar, vanilla, and salt; mix until smooth.
Beat in the cooled strawberry reduction, then add more powdered sugar, 1/2 cup at a time, until fluffy and pipeable. Add a splash of lemon juice if you want extra brightness.
Frost: Pipe or spread frosting onto cooled cupcakes. Garnish with a strawberry slice or tiny lemon wedge if you like.