Instructions

for the crêpes

 Heat oil in 8-inch nonstick skillet over low heat for at least 10 minutes. Meanwhile, process hazelnuts in food processor until finely ground, about 1 minute; transfer to large bowl. Whisk in flour, sugar, and salt until combined. Whisk milk and eggs together in second bowl. Whisk half of milk mixture into dry ingredients until smooth. Add melted butter and whisk until incorporated. Whisk in remaining milk mixture until smooth.
 Wipe out skillet with paper towel, leaving thin film of oil on bottom and sides of pan. Increase heat to medium and let skillet heat for 1 minute. After 1 minute, test heat of skillet by placing 1 teaspoon batter in center and cooking for 20 seconds. If mini crêpe is golden brown on bottom, skillet is properly heated; if it is too light or too dark, adjust heat accordingly and retest.
 Pour ¼ cup batter into far side of skillet; tilt and shake gently until batter evenly covers bottom of skillet. Cook crêpe without moving it until top surface is dry and edges are starting to brown, loosening crêpe from sides of pan with heat-resistant rubber spatula, about 1 minute. Gently slide spatula underneath edge of crêpe, grasp edge with your fingertips, and flip crêpe. Cook until second side is lightly spotted, about 30 seconds. Transfer crêpe to wire rack, inverted so spotted side is facing up. Return pan to heat and heat for 10 seconds before repeating with remaining batter. (You should have about 24 crêpes.) As crêpes are done, stack on wire rack. Let cool completely. (Crêpes can be stacked, covered with plastic wrap, and refrigerated for up to 24 hours; bring to room temperature before using.)

for the filling

Microwave chocolate in bowl at 50 percent power, stirring occasionally, until melted and smooth, 2 to 4 minutes; let cool slightly. Using stand mixer fitted with paddle, beat butter, melted chocolate, and salt on medium speed until smooth, about 1 minute. Reduce speed to low and slowly add sugar. Increase speed to medium and beat until smooth, about 2 minutes, scraping down bowl as needed. Add marshmallow crème, increase speed to medium-high, and beat until light and fluffy, 3 to 5 minutes. Reduce speed to low, add cocoa and hazelnut extract, and beat to incorporate, about 30 seconds. (Filling can be refrigerated for up to 3 days; let soften at room temperature, about 2 hours, then rewhip on medium speed until smooth, 2 to 5 minutes.)

for the whipped cream

 Using clean, dry mixer bowl and whisk attachment, whip cream, sugar, and vanilla on medium-low speed until foamy, about 1 minute. Increase speed to high and whip until soft peaks form, 1 to 3 minutes. (Whipped cream can be refrigerated in fine-mesh strainer set over small bowl and covered with plastic wrap for up to 8 hours.)
 Place small dab of filling in center of cake platter to anchor cake. Lay 1 crêpe on platter. Spread 3 tablespoons filling evenly over top, right to edge of crêpe. Top with 1 crêpe, pressing lightly to adhere. Repeat with remaining crêpes and filling, refrigerating cake for 15 minutes halfway through assembly to set layers and ending with crêpe on top. Refrigerate for at least 30 minutes or up to 12 hours.
 Spread whipped cream into even layer on top of cake, then garnish with chopped hazelnuts. Let stand at room temperature for 20 minutes before serving.