TO MAKE THE CREPES: Combine all crepe ingredients in a blender. Puree until smooth and thoroughly combined, about 1 minute. Transfer to a bowl, cover, and refrigerate until cold, at least 2 hours and up to overnight.
When ready to cook the crepes, vigorously stir batter until smooth again.
Melt 1/2 teaspoon butter in a 10-inch nonstick skillet over medium heat.
Tilt skillet at a 45-degree angle, pour 1 ounce batter (slightly more than 2 tablespoons) into skillet, and immediately swirl and shake skillet in a circular motion to evenly distribute batter in a thin film across bottom. Cook until edges of crepe turn golden, about 45 seconds.
Carefully flip crepe, and cook just until set, about 30 seconds. Transfer to a paper-towel-lined plate.
Melt another 1/2 teaspoon butter, and continue cooking crepes in the same manner, mixing batter with a spoon or ladle between crepes and stacking cooked crepes on top of one another (you should have 20 to 22 crepes).
Let crepes cool completely, about 30 minutes.
TO MAKE THE LEMON CURD:
Beat together butter and sugar with a mixer on medium-high speed until light and fluffy. Reduce speed to low, and add whole egg.
When thoroughly combined, gradually add egg yolks, then lemon juice (the mixture will appear curdled).
Transfer mixture to a small saucepan, and cook, stirring constantly, over medium heat until thick enough to coat the back of a wooden spoon, 5 to 7 minutes.
Transfer to a bowl, cover surface of curd with plastic wrap, and let cool completely in the refrigerator, about 2 hours.
TO ASSEMBLE THE CAKE:
Whisk together 1 1/4 cups cream and 3 tablespoons sugar until soft peaks form.
Fold a third of the whipped cream into lemon curd with a rubber spatula until thoroughly combined.
Gently fold cream-curd mixture into remaining whipped cream until thoroughly combined.
For a neat presentation, make a frame on a 12-inch cake plate with 2-inch-wide strips of parchment.
Place 1 crepe in center of plate (the outer edge of plate should be covered with parchment), and spread 3 tablespoons curd mixture evenly over crepe. Repeat layering with remaining crepes and curd mixture, ending with the last crepe.
Whisk together remaining 1 1/4 cups cream, the mascarpone, the remaining 3 tablespoons sugar, and the vanilla until soft peaks form.
Spoon mascarpone cream on top of cake, and carefully spread across top and sides.
Refrigerate until set, about 2 hours.
Let stand at room temperature 30 minutes. Just before serving, remove parchment frame, and garnish with amaretti cookies.