For the cake:
4 1/2 ounces (1 cup) cake flour
1 tablespoon cornstarch
1 teaspoon baking powder
1 teaspoon ground cinnamon
1 teaspoon ground ginger
1/4 teaspoon ground nutmeg
1/4 teaspoon salt
3 large eggs
1 cup packed dark brown sugar
2/3 cup pumpkin purée (I prefer Libby’s)
1 teaspoon fresh lemon juice
1 teaspoon vanilla extract
Powdered sugar, for rolling
For the hazelnut syrup:
2 tablespoons sugar
1 tablespoon hazelnut liqueur (such as Frangelico) or vanilla extract
1 tablespoon water
For the buttercream:
1 cup sugar
4 large egg whites
1 1/2 cups (12 ounces) unsalted butter, cubed and softened
2 tablespoons hazelnut liqueur (such as Frangelico) or vanilla extract
To make the cake: Preheat oven to 375°F. Generously coat a 15- by 10-inch jellyroll pan with baking spray. Combine the flour, cornstarch, baking powder, cinnamon, ginger, nutmeg, and salt in a medium mixing bowl.
Place the eggs in the bowl of a stand mixer fitted with a beater blade (or with a electric hand mixer) and mix on medium-high speed until thick and pale. Slowly add the brown sugar and continue beating for five minutes. Turn the mixer off and fold in the pumpkin, lemon juice, and vanilla.
Gently, but firmly, fold the dry ingredients into the batter until completely combined. Pour the batter into the prepared pan and smooth out with a large offset spatula, paying particular attention to the edges. Bake for 10 to 12 minutes, until just firm to the touch. (When in doubt, err on the earlier side.)
Place a clean, thin kitchen towel (preferably linen) that is larger than the jellyroll pan on a work surface. Sift a liberal amount of powdered sugar all over. Run a knife around edges of the cake and immediately invert the still-warm cake onto the towel and generously sift more powdered sugar over the top. (This prevents the towel from sticking.) Starting with a narrow end, tightly roll up the cake and towel into a log. Let cool to room temperature, seam-side down. (This allows the cake to form a "shape memory” and prevents cracking.)
To make the sugar syrup and buttercream: While the cake is cooling, make the sugar syrup by combining sugar, hazelnut liqueur, and water in a small microwave-safe bowl and heating until sugar is dissolved, 40 to 60 seconds. Set aside to cool.
For the buttercream: Place the sugar and egg whites in the bowl of a stand mixer fitted with a whisk attachment and place over (not in) a saucepan of simmering water. Whisk frequently until the mixture