How to Make It
Whisk together flour, sugar, lemon zest, and salt in a large bowl. Process eggs, milk, and heavy cream in a blender until well combined, about 30 seconds. Add flour mixture to egg mixture; process until smooth and frothy, about 1 minute, stopping to scrape down sides of blender as needed. Transfer batter to a container with a tight-fitting lid; seal and chill at least 4 hours or up to overnight.
Remove batter from refrigerator; gently whisk to reincorporate any bubbles or separation.
Brush an 8-inch nonstick skillet with some of the melted butter, evenly coating bottom of skillet. Cook over medium until butter is hot, 1 to 2 minutes. Remove from heat. Using a 1/4-cup measuring cup, immediately add a scant 1/4 cup batter to one side of skillet. Swirl skillet to spread batter in a thin, even layer.
Return skillet to heat; cook over medium until crêpe edges are light golden and set, about 1 minute. Using a small offset spatula, gently lift one edge of the crêpe; using your fingertips, quickly flip crêpe. Cook until other side of crêpe is pale golden and a few spots begin to appear on bottom, 30 to 45 seconds. Transfer crêpe to a wire rack set over a rimmed baking sheet to cool; cover with a clean, dry lightweight towel.
Repeat steps 3 and 4 with remaining butter and batter, wiping skillet clean and lightly brushing with butter every few crêpes, or as needed. (You should have 20 crêpes to use for cake. Reserve any extra crêpes for another use.) Chill crêpes, covered, until ready to use.
Combine mascarpone, 1/4 cup heavy cream, 1/4 cup powdered sugar, and lemon zest in bowl of a stand mixer; gently whisk by hand until incorporated. Add remaining 1 pint heavy cream; fit mixer with whisk attachment. Beat mixture on medium-high speed, gradually adding remaining 1/4 cup powdered sugar, until stiff peaks form, 1 to 2 minutes.
Mash 3/4 cup blackberries in a medium bowl. Add about half of the mascarpone whipped cream to mashed berries, and fold gently to combine.
Remove crêpes from refrigerator. Place 1 crêpe on a cake platter or flat serving dish. Using a small offset spatula, gently spread a thin layer (about 3 tablespoons) of the plain mascarpone whipped cream over crêpe, leaving a 1/4-inch border. Top with 1 crêpe, and gently spread with a layer (about 3 tablespoons) of the blackberry mascarpone whipped cream. Repeat process with remaining 18 crêpes, alternating with layers of plain mascarpone whipped cream and blackberry mascarpone whipped cream, until you have used the final crêpe (Do not spread anything on top layer). Chill remaining plain mascarpone whipped cream until ready to use. Using kitchen shears, trim any excess crispy crêpe edges that may hang over. Cover cake with plastic wrap; chill at least 1 hour and up to 8 hours.
Remove cake from refrigerator, and uncover. Spoon remaining plain mascarpone whipped cream over top of cake, creating a small well about 6 inches wide in the center. Pile remaining 1 1/4 cups blackberries on top. Slice cake using a serrated knife, and serve.