In a large heatproof bowl, whisk egg yolks, cornstarch, salt, and ½ cup granulated sugar until combined.

In a medium pot over medium heat, heat milk and remaining ½ cup granulated sugar, stirring, until sugar is dissolved and milk is steaming. Whisking constantly, pour half of hot sugar milk into egg mixture. Pour milk-egg mixture into pot and cook over medium heat, stirring constantly, until pastry cream boils and thickens to a pudding-like consistency, 1 to 2 minutes. Cook, stirring, 1 minute longer, then remove from heat. Stir in butter and vanilla.

Pour hot pastry cream onto a rimmed baking sheet and cover with plastic wrap, pressing directly onto cream. Refrigerate until cold.

In a large bowl, whisk heavy cream until stiff peaks form. Transfer cold pastry cream to another large bowl and stir until smooth. Stir one-quarter of whipped cream into pastry cream until combined. Using a rubber spatula, carefully fold in remaining whipped cream. Refrigerate pastry cream filling until ready to assemble.

Make Ahead: Pastry cream (without the whipped cream folded in) can be made 3 days ahead. Store in an airtight container and refrigerate.

In a blender, blend milk, eggs, flour, vanilla, melted butter, salt, and baking powder until smooth, about 1 minute. Cover and refrigerate at least 1 hour and up to 2 days.

In a large nonstick skillet over medium heat, melt remaining 1 tsp. butter. Whisk batter, then pour ⅓ cup into center of pan, swirling pan to spread out to an even 9" circle. Cook until top is set and bottom is golden brown, 1 to 2 minutes. Flip and cook second side until cooked through, about 1 minute more.

Transfer crêpe to a parchment-lined wire rack; cover with a half sheet of parchment. Repeat with remaining batter until you get 18 to 20 crêpes. Let cool.

Make Ahead: Crêpes can be made 1 day ahead. Stack between sheets of parchment. Store in an airtight container and refrigerate.

Arrange 1 crêpe in the center of a platter. Spread a scant ½ cup pastry cream filling over top, leaving a ½" border around the edges. Top with another crêpe. Repeat until all crêpes are stacked, saving a nice crêpe for the top.

Cover and refrigerate at least 4 hours or up to overnight.

In a large bowl, whisk heavy cream, confectioners' sugar, and vanilla until stiff peaks form. Spread on top of cake, swirling to create peaks and curves. Top with berries.