Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper cupcake liners.
In a medium bowl, sift together the flour, baking powder, and salt. Set aside.
2½ cups (312 g) all-purpose flour, 2¼ tsps. (9 g) baking powder, ¼ tsp (1 g) table salt
In a large bowl, use an electric mixer to cream the softened butter and granulated sugar until fluffy and pale, about 3 minutes.
12 tbsps. (170 g) unsalted butter, 1¼ cups (250 g) granulated sugar
In a separate bowl with clean beaters, beat the egg yolks and vanilla extract until thick and light in color, about 4 minutes.
8 large egg yolks, 1 tsp (5 ml) vanilla extract
Add the egg yolk mixture to the butter-sugar mixture and mix to combine.
With the mixer on low, add one-third of the dry ingredients, then half the milk. Repeat with another third of the flour, the remaining milk, and the last of the flour. Mix until just combined.
1 cup (240 ml) whole milk
Scrape the sides and bottom of the bowl with a spatula, then gently fold in the rainbow jimmies.
½ cup (80 g) "jimmy" rainbow sprinkles
Divide the batter evenly into the muffin cups and bake for 22–25 minutes, or until a toothpick inserted in the center comes out clean. Let cool in the pan briefly, then transfer to a wire rack to cool completely.
Just before serving, combine the heavy cream, confectioners’ sugar, and vanilla in a large bowl. Beat with an electric mixer until thick and soft peaks form.
2 cups (480 ml) heavy cream, ½ cup (60 g) confectioners' sugar, 1 tsp (5 ml) vanilla extract
Pipe or spoon whipped cream onto each cupcake, then top with blueberries and raspberries.
¾ cup (115 g) fresh blueberries, ¾ cup (95 g) fresh raspberries
Sprinkle generously with Pop Rocks just before serving, or serve with individual packets so guests can add their own.
6-12 packets Pop Rocks*