Preheat oven to 350°F (175°C) and line a cupcake pan with liners.
Whisk together flour, cocoa powder, baking soda, and salt in one bowl.
Mix sugar and vegetable oil in a separate bowl until combined.
Add egg, milk, and vanilla extract to the sugar mixture; whisk until smooth.
Gradually add dry ingredients to wet ingredients and mix until just combined.
Divide batter evenly among cupcake liners, filling each about ⅓ full.
Bake for 18–20 minutes, until a toothpick inserted comes out clean.
Remove from oven and let cupcakes cool completely before frosting.
Beat softened butter until creamy and smooth.
Gradually add powdered sugar, mixing well after each addition.
Add strawberry purée and vanilla extract.
Beat until light and fluffy, about 2–3 minutes.
Add food coloring if you want a pink or red color.
Pipe or spread strawberry buttercream onto cooled cupcakes.
Garnish with fresh strawberry slices, chocolate shavings, or heart sprinkles if you like.
Chill for 10–15 minutes for a cleaner presentation.