You won't believe how delicious this French gateau Breton is. Well, it's really a cross between shortbread and cake, something that is very common for European cakes.
Preheat the oven to 375 degrees F (190 degrees C). Grease a 9-inch springform pan.
Mix 1 teaspoon egg yolk and water in a small bowl. Set aside for glazing the cake.
Combine remaining egg yolks, flour, butter, sugar, and cornstarch in a large bowl. Mix with an electric mixer until dough is stiff and sticky, similar to cookie dough. Knead dough a little bit to bring it together.
Press dough into the prepared pan with your hands. Brush glaze on top.
Bake in the preheated oven for 15 minutes. Reduce temperature to 350 degrees F (175 degrees C). Continue baking until golden brown, about 25 minutes more. Cool for 10 minutes. Slice cake crosswise.