Dice the strawberries small. Toss with 1 tablespoon sugar and let sit 10 minutes. This draws out juice and softens the fruit so it blends into the batter without sinking.
Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners. Room-temperature ingredients mix more evenly, so set eggs and buttermilk out early.
In a large bowl whisk flour, sugar, baking powder, and salt until evenly combined. This prevents pockets of leavener.
In a second bowl whisk oil, eggs, buttermilk, lemon juice, lemon zest, and vanilla until smooth. The mixture should look slightly emulsified.
Pour the wet ingredients into the dry. Stir gently with a spatula just until no dry streaks remain. Fold in the macerated strawberries and any accumulated juice. Batter will be thick but pourable. Do not overmix—overworked gluten makes tough cupcakes.
Divide batter evenly among the liners, filling each about two-thirds full. Bake 18–20 minutes until the tops spring back when lightly pressed and a toothpick inserted in the center comes out clean or with a few moist crumbs. Edges should look lightly golden.
Let cupcakes rest in the pan 5 minutes, then transfer to a wire rack. Cool fully before frosting so the buttercream does not melt.
Beat softened butter on medium speed until creamy, about 2 minutes. Add powdered sugar one cup at a time, mixing on low. Scrape the bowl, then add lemon juice, strawberry purée, and salt. Beat on medium-high 2–3 minutes until light and fluffy. Adjust with a splash more lemon juice if too thick or a spoonful more sugar if too soft.
Pipe or spread frosting onto cooled cupcakes. Top with a thin strawberry slice and a pinch of lemon zest. Serve at room temperature for the best texture.