Prep the pan and oven. Heat the oven to 350°F (180°C). Line a 9x13-inch (23x33 cm) baking pan with parchment, leaving an overhang on two sides. Lightly grease the parchment and sides. This helps the cake release cleanly.
Whisk dry ingredients. In a medium bowl, whisk 2 1/2 cups all-purpose flour with 2 1/2 teaspoons baking powder and 1/2 teaspoon fine salt. Set aside.
Cream the butter and sugar. In a large bowl, beat 3/4 cup unsalted butter with 1 1/4 cups granulated sugar until pale and fluffy, about 2–3 minutes. Proper creaming builds structure and gives you a soft crumb.
Add eggs and vanilla. Beat in 3 large eggs one at a time, scraping the bowl between additions. Mix in 2 teaspoons vanilla extract. If the mixture looks slightly curdled, that’s fine—it will smooth out when you add dry ingredients.
Combine dairy and fold in dry mix. Stir together 1/2 cup sour cream and 3/4 cup whole milk. Add the dry ingredients to the butter mixture in two additions, alternating with the dairy mixture, beginning and ending with dry. Mix on low just until combined. Do not overmix.
Fold in sprinkles. Gently fold in 1/2 cup rainbow jimmies. Use a light hand to prevent color streaking. Avoid nonpareils; they bleed more in batter.
Bake. Transfer the batter to the prepared pan and smooth the top. Bake for 25–32 minutes, until the top is golden and a toothpick comes out with a few moist crumbs. Rotate once if your oven bakes unevenly.
Cool completely. Let the cake cool in the pan for 15 minutes, then use the parchment overhang to lift it onto a wire rack. Cool fully before frosting—warm cake will melt the icing.
Make the frosting. Beat 3/4 cup unsalted butter until creamy. Add 3 cups confectioners’ sugar, 1 teaspoon vanilla, a pinch of salt, and 2–3 tablespoons heavy cream. Beat on medium-high for 2 minutes, until fluffy. For brightness, add 1 teaspoon lemon zest. Adjust consistency with more sugar (thicker) or cream (softer).
Color and decorate. Leave the frosting white or tint with a drop of pastel food coloring. Spread evenly over the cooled cake. Top with more jimmies, crushed mini eggs, or small candy shapes. For a marbled look, dot the top with tiny blobs of a second color and swirl lightly with an offset spatula.
Slice and serve. Cut into 16–24 squares, depending on the occasion. The cake is tender but sturdy, so it’s great for buffets and lunchboxes.
