• Preheat oven to 350°F (180°C). Spray 3 (8-inch) round cake pans with baking spray with flour. Line bottom of pans with parchment paper.
• In the bowl of a stand mixer fitted with the paddle attachment, beat butter and brown sugar at medium speed until fluffy, 2 to 3 minutes, stopping to scrape bottom and sides of bowl. Add eggs, one at a time, beating well after each addition. Beat in vanilla.
• In a large bowl, whisk together flour, cornstarch, baking powder, and salt. Add flour mixture to butter mixture; beat at low speed until almost combined. Fold in jimmies until combined. Divide dough among prepared pans (about 475 grams each), pressing into an even layer.
• Bake until golden brown and a wooden pick inserted in center comes out clean, 20 to 25 minutes, rotating pans halfway through baking. Let cool in pans for 10 minutes. Remove from pans, gently remove parchment, and let cool completely, flat side down, on wire racks.
• Place 1½ cups (255 grams) Brown Sugar Swiss Meringue Buttercream in a pastry bag fitted with a large open star piping tip (Ateco #826); reserve. Place 1 cooled cookie layer, flat side down, on a serving plate. Spread 1½ cups (255 grams) buttercream onto cookie. Top with second cookie layer, flat side up; spread 1½ cups (255 grams) buttercream onto cookie. Top with remaining cookie layer, flat side down.
• Using a large offset spatula, spread 1 cup (170 grams) buttercream on top of cake. Spread a thin layer of buttercream on sides of cake. Pipe reserved buttercream around top edge of cake as desired. Garnish with jimmies, if desired. Refrigerate for at least 30 minutes before serving; cover and refrigerate for up to 3 days.

Brown Sugar Swiss Meringue Buttercream

• In the heatproof bowl of a stand mixer, whisk together brown sugar and egg whites by hand. Place bowl over a saucepan of simmering water. Cook, whisking frequently, until sugar dissolves and an instant-read thermometer registers 150°F (66°C) to 160°F (71°C).
• Carefully return bowl to stand mixer. Using the whisk attachment, beat at medium-high speed until mixture has cooled and stiff peaks form, 4 to 5 minutes. With mixer on medium speed, add butter, 1 tablespoon (14 grams) at a time, beating until combined after each addition. Beat in salt and vanilla. Increase mixer speed to medium-high, and beat until buttercream is thick and smooth. (It might look curdled at first, but it will come together.) Use immediately.