Preparation

Bake the cake

• Position a rack in the center of the oven and heat the oven to 350°F. Coat a 10×15-inch jelly roll pan with cooking spray. Line with parchment paper, and spray the parchment as well.
• In a medium bowl, whisk the flour, cinnamon, ginger, nutmeg, baking powder, salt, and baking soda.
• In a stand mixer fitted with the whisk attachment or with a hand-held mixer, beat the sugar and oil on medium speed until smooth, about 1 minute. Add the eggs one at a time, beating well after each addition, and then add the vanilla. Reduce the speed to low, and mix in the carrots. Fold in the dry ingredients by hand with a spatula. Stir in the nuts.
• Pour the batter into the prepared pan, spreading it evenly. Tap the pan on the counter to settle the batter. Bake until a toothpick inserted into the center comes out clean, 18 to 20 minutes, turning the pan halfway through baking time. Transfer to a cooling rack and let cool completely in the pan.

Make the candied carrot curls

• Line two rimmed baking sheets with parchment and lightly coat one with cooking spray. Heat the oven to 225°F. combine the sugar with 1-1/2 cups water in a 3-quart saucepan, and cook over medium heat until the sugar dissolves.
• Meanwhile, shave the carrots into 6- to 7-inch strips with a vegetable peeler. Simmer in the sugar syrup until translucent, about 15 minutes. Transfer with a slotted spoon to the oiled baking sheet. Bake until dried but still pliable, 30 minutes.
• As soon as you can handle them, curl the strips around the handle of a wooden spoon so that the side that was on the oiled sheet is in contact with the spoon. Hold for a few seconds; then slide the curl onto the unoiled baking sheet. Bake until set, another 5 to 8 minutes.

Make the candied pecans

• Line a small rimmed baking sheet with parchment. Cook the sugar in a 10-inch nonstick skillet over medium-low heat without stirring until melted and golden, about 3 minutes. Add the nuts and cook, tossing occasionally, until the sugar is dark amber, 2 to 3 minutes. Transfer the nuts to the prepared baking sheet to cool completely. Chop coarsely.

Make the frosting

• In a stand mixer fitted with the whisk attachment or with a hand-held mixer, beat the mascarpone, cream, sugar, and vanilla on medium speed until smooth, about 1 minute. Increase the speed to medium high, and beat until the frosting holds stiff peaks, 30 to 60 seconds. Do not overbeat, or the frosting will lose its luster.

Assemble the cake

• Use an offset spatula to loosen the cake from the sides of the pan. Invert it onto a cutting board, and gently peel off the parchment paper. Using a serrated knife, cut the cake in half crosswise, then cut each half again to get four equal rectangular pieces.
• Place one piece of cake, top side down, on a serving platter. Tuck strips of waxed paper or parchment under its edges. Spread a heaping 1/2 cup of frosting evenly over the layer with the offset spatula.
• Place a second layer, top side up, on top. Make sure the edges are aligned. If it looks uneven, apply slight pressure to even it. Spread with another heaping 1/2 cup frosting. Repeat with the third layer, top side up, and another heaping 1/2 cup frosting. Top with the fourth layer, top side up.
• Put about 1-1/2 cups of frosting in a small bowl. Spread this frosting in a thin layer over the top and sides of the cake to seal in the crumbs; don’t worry if this frosting is speckled with crumbs. Chill the cake in the refrigerator for 5 minutes.
• Spread the remaining frosting over the top and sides of the cake (you may not need it all). Refrigerate the cake for at least 3 hours and up to 12 hours. (Cover the cake loosely with plastic if storing for more than 3 hours.)

Garnish and serve

• Just before serving, top the cake with the candied nuts and carrots, if using. Use a serrated knife to slice the cake.

Make Ahead Tips

The cake can be baked 1 day ahead and kept covered at room temperature.

The candied carrot curls and nuts can be made up to one day ahead and kept in an airtight container at room temperature.

The cake can be frosted up to 12 hours before serving.