Preheat the oven to 350F/177C. Line 18 cupcake tins with paper liners, or spray the insides with a baking spray.
Into a large mixing bowl, add the wet ingredients plus the sugar: eggs, milk, oil, sugar, vanilla and strawberry extract. Using a mixer or a hand whisk, whisk on high speed for 2 to 3 minutes, until the mixture is smooth and the eggs are well beaten. In a separate bowl, combine the dry ingredients: flour, baking powder and salt. Sift the dry ingredients into the cake batter. Mix on medium speed for about 1 minute, just until the dry ingredients are incorporated. Watch my video recipe to see how it's done.
Next, gently fold in the diced strawberry. Using a measuring scoop, fill each cupcake tin about 1/2 to 3/4 of the way full. Bake in the preheated oven for 18 to 22 minutes, until the tops of the cupcakes are golden brown. Remove them from the oven and let the cupcakes cool completely on a wire rack.
Making the Strawberry Buttercream:
Meanwhile, prepare the buttercream. Place the softened butter into a mixer bowl and whisk on high speed for 7 to 9 minutes, scraping down the sides of the mixing bowl often, until the butter is light, fluffy and pale white in color.
Next, add the confectioner's sugar, strawberry extract and salt. Add more sugar for a thicker frosting. Start mixing the sugar into the butter on a low speed, then turn up to high speed, and continue whisking for another 2 to 3 minutes. The buttercream should be very light and fluffy! Scrape down the sides of your mixing bowl often.
Next, add in the berry preserves. Mix the preserves into the buttercream on high speed for 15 to 20 seconds, just until they're incorporated; do not overmix!!
Assembling the Cupcakes:
Transfer the prepared buttercream into a pastry bag tipped with star tip; I used Ateco tip #847. Pipe generous dollops of buttercream onto completely cooled cupcakes. Garnish the top with small strawberries, if desired. Store the cupcakes in the refrigerator if not enjoying right away.