Mother's Day Sheet Cake
Combine cream and confectioners' sugar in the bowl of an electric mixer fitted with the whisk attachment. Scrape in vanilla seeds; reserve pods for another use. Starting on low speed and increasing to medium high, whip until stiff peaks form, about 2 minutes.
Invert one 13-by-18-inch cake onto a parchment paper-lined cutting board, and peel parchment paper from bottom of cake. Using a serrated knife, trim 1/4 inch from short sides of cake. Cut in half crosswise. Repeat with second cake. Place one cake half on a serving platter. Using an offset spatula, spread one-fourth of the whipped cream over cake layer; arrange one-fourth of berries over cream. Repeat process with remaining cake halves, cream, and berries. Refrigerate until serving; garnish with mint.