The tart sweetness of the raspberry curd filling balances this buttery-rich, French-style sponge cake. Martha made this recipe on Martha Bakes episode 601.
Make the cakes: Preheat oven to 350 degrees. Butter two 8-by-2-inch round baking pans; line bottoms with parchment paper rounds. Butter parchment; dust pans with flour, tapping out excess.
In a small saucepan, melt butter over medium-high heat until nut-brown in color, about 8 minutes. Remove pan from heat and pour butter into a large bowl, leaving behind any burned sediment; stir in vanilla and keep warm (110 to 120 degrees). Wipe out saucepan.
Fill the same saucepan with a few inches of water and bring to a simmer. Lightly whisk together eggs and superfine sugar in a heatproof standing mixer bowl. Set bowl over (not in) simmering water, and whisk until superfine sugar dissolves and mixture is lukewarm to the touch.
Attach bowl to standing mixer fitted with the whisk attachment and whisk on high speed until light, airy, and quadruple in volume, at least 5 minutes. Whisk 1 cup egg mixture into warm butter mixture.
Sift together flour and cornstarch. Gently and quickly fold half the flour mixture into remaining egg mixture until combined, then repeat with remaining flour mixture. Gently fold in butter mixture, then divide between prepared baking pans.
Transfer pans to oven and bake until golden brown and edges begin to pull away from the sides, about 25 minutes. (Avoid opening oven door during baking, as cakes are fragile and may collapse.) Remove cakes from oven and unmold immediately onto a wire rack to cool completely.
Make the raspberry curd filling: In a small food processor, puree raspberries until smooth.
Whisk together granulated sugar and cornstarch in a medium saucepan. Whisk in zest and egg yolks, then whisk in lemon juice and salt. Add butter and place pan over medium-high heat. Cook, whisking constantly, until butter has melted, small bubbles form around the edge of the pan, and mixture is thick enough to coat the back of a spoon and becomes clear, about 5 minutes.
Remove pan from heat. Whisk in raspberry puree. Strain mixture through a fine mesh sieve into a glass bowl. Press plastic wrap directly onto surface of curd and refrigerate until cool, about 30 minutes.
Assemble the cake: Using a serrated knife, halve each cooled cake horizontally to form 2 layers. Place 1 layer on a cake turntable or serving platter; spread a scant 1/2 cup curd on top. Set another cake layer on top, and repeat process with remaining curd and cake layers. Refrigerate until firm, about 30 minutes.
Make the whipped cream: Whisk cream and confectioners' sugar to soft peaks. Spread entire cake with whipped cream, smoothing the top and sides. Top with raspberries, if desired.