Preheat oven to 355ºF | 180ºC and grease a 9-inch | 23cm round pan with butter.
Pulse walnuts in a food processor, leaving some for decoration.
Separate the egg yolks from the whites. Beat egg whites until soft peaks form and set aside.
Beat sugar with butter with a hand mixer until you have a white, pasty mixture. Add in the egg yolks, pulsed nuts and cornstarch and mix until combined.
Using a spatula (not the mixer), gently incorporate the egg white peaks until they are fully integrated in the mixture.
Pour the cake batter in the mold, decorate with walnuts and place in the oven for 35 minutes.
Unmold when cake is no longer hot.
Notes
According to Monash University, large servings of walnuts (4,8 oz | 135g) contain moderate amounts of oligos-fructans and intake should be limited. This means you can eat up to 8 slices per meal.