Preheat the oven to 350°F, and line cupcake pan with cupcake liners.: You will notice the slight warm scent of the oven as it comes up to temperature, which primes the baking environment and helps the cupcakes rise evenly. Preheating prevents the batter from sitting and overdeveloping gluten while you wait, which would make the crumb tough. A common mistake is skipping preheat, which leads to uneven rise and dense centers. Make sure the oven rack is centered, and use liners that fit snugly so the batter holds its shape while baking.
Cream together butter and sugar until light and pale in a large bowl with an electric mixer. About 2 minutes.: As you cream, you should hear a steady, soft hum from the mixer and see the mixture lighten in color to a pale yellow, indicating air is being incorporated for a tender crumb. The aroma will turn rich and buttery. If you under-cream, the cupcakes may be denser and not rise as evenly, while over-creaming can cause the batter to collapse later. Scrape the bowl to ensure even mixing, and stop when the texture looks fluffy and aerated.
Add the eggs, lemon zest and juice, vanilla, salt, flour and baking powder. Mix at medium speed for 2 minutes, until well combined. Then fold in the raspberries.: After adding the wet and dry components, you should see a cohesive batter that is smooth but not overworked, with tiny flecks of lemon zest visible. The batter will smell bright and slightly sweet. Folding in the raspberries gently preserves their shape and prevents the batter from turning pink and thin. Overmixing here develops gluten and yields a tougher texture, so stop as soon as streaks of flour are incorporated and the fruit is evenly distributed.
Divide the batter evenly in the cupcake liners, to the 2/3 level.: As you spoon or scoop the batter, the surface should be glossy and thick, holding small air pockets. Filling to the two thirds level gives enough room for rise without overflow. An uneven fill leads to cupcakes that bake at different rates, so use a scoop to portion consistently. If the batter looks overly runny, you might have overmixed or added too much liquid; chill briefly if needed to firm it up before filling.
Bake for 19-24 minutes, until golden. Transfer to a wire cooling rack to cool.: During baking, watch for a light golden top and a scent of warm citrus and baked butter. A toothpick inserted should come out with a few moist crumbs, not wet batter. Leaving cupcakes in the pan too long steams them and softens the crust, while taking them out too soon risks underbaking. Shift them to a wire rack immediately to stop cooking, and let them cool completely before frosting to prevent the buttercream from melting.
For the buttercream, put the raspberries and lemon juice in a small saucepan over low heat. Heat them slowly until they begin to break down, then mash them with a fork, and simmer for 10 minutes. Press the cooked raspberries through a sieve to get the juice, and allow to cool.: As the berries soften, the kitchen will fill with a vivid, fruity aroma and the mixture will reduce to a concentrated, fragrant syrup. Sieving removes seeds and creates a smooth base for the frosting. If you skip simmering long enough, the frosting can taste raw or overly seedy; if you boil aggressively, you may lose bright aromatics. Keep the heat low and stir so nothing scorches, then cool fully to avoid melting the butter in the next step.
Beat the softened butter in a mixer for 5 minutes, until pale. Add the confectioners’ sugar in 2 stages, mixing for 2 minutes each time. When fully mixed, gradually add the raspberry juice and mix until the butter cream is pink, and fluffy in consistency.: The butter will first look glossy, then shift to a pale, airy texture as you whip, releasing a clean buttery scent. Adding the powdered sugar in stages prevents dusting and helps the frosting cream smoothly. When you pour in the raspberry juice slowly, the color should bloom and the mixture will become lighter and more fragrant. If the frosting is too loose, add a bit more sifted sugar; if too stiff, a teaspoon of cooled raspberry juice will soften it. Avoid adding hot juice or the frosting can separate.
Pipe the icing on top of the cupcakes, and top each with a fresh raspberry, a sprinkling of freeze-dried raspberries if desired.: The moment you pipe, the buttercream should hold its shape and have a satiny sheen, giving a pleasing contrast to the tender cake. Pressing a fresh raspberry on each cupcake adds a bright visual and juicy burst when eaten. If the frosting droops, it may have been overworked or the cupcakes were still warm. Chill briefly to firm up, then finish decorating. For a delicate finish, dust with crushed freeze dried raspberries to add color and a slight crunch.