Yield: 1 roulade
For cake, preheat oven to 350° F. Line a 15 x 10-inch jelly roll pan with parchment paper, grease and flour, being certain to shake off excess. In a mixer fitted with the whisk attachment or with electric beaters, whip egg yolks and ¾ cup sugar until they hold a ribbon when whisk is lifted. Stir in melted butter, molasses, vanilla and lemon zest. In a separate bowl, sift flour, cornstarch, baking powder, nutmeg, cinnamon and salt and fold into egg yolk mixture. Whip egg whites with cream of tartar until frothy, then add 2 Tbsp sugar and continue whipping to stiff peak. Fold a third of whites into batter until almost incorporated, then fold in remaining two thirds. Spread onto prepared pan and bake for 15 to 18 minutes, until cake springs back when gently pressed.
Remove cake from oven and loosen edges of cake from pan. Dust cake generously with icing sugar, place a sheet of parchment over to cover, top with another pan (if you have one) and flip over onto work area. Peel off parchment from jelly roll and dust top generously with icing sugar. Starting from a long side, roll up cake with parchment and let cake cool. This will set the jelly roll shape.
For filling, whip cream and icing sugar to soft peaks. Stir in lemon juice and zest into mascarpone. Fold cream into mascarpone mixture.
To assemble, unroll sponge cake and spread with filling, leaving a gap of about 2 to 3 inches at the end. Roll up cake, being gentle so as not to push out filling (but some will move forward). Dollop remaining filling on top along length of cake, dust with icing sugar and garnish with lemon curls.