Start by preheating your oven to 350°F (175°C). Butter a 9-inch tart or springform pan thoroughly, then dust it lightly with flour, tapping out any excess. Set aside. This step ensures easy release and even baking.

In a large bowl, beat the softened butter and sugar together with an electric mixer on medium speed for about 3 to 4 minutes. The mixture should become pale and creamy. Proper creaming is essential for the rich and tender texture of this cake.

Add the 4 egg yolks, one at a time, beating well after each addition. Make sure the yolks are fully incorporated before adding the next. Scrape down the sides of the bowl as needed. This step builds the dense and rich body typical of Breton butter cake.

In another bowl, whisk together the all-purpose flour, cake flour, baking powder, and salt. Mixing these dry ingredients beforehand ensures even distribution in the dough.

Gradually add the dry ingredients into the butter mixture, mixing on low speed just until a soft, thick dough forms. Do not overmix. The dough should be smooth but firm, almost like a cookie dough.

Transfer the dough into the prepared pan. Press it in evenly using a spatula or clean hands. Smooth out the top. Brush the surface lightly with the beaten whole egg. For the classic look, use the back of a fork to gently score a crosshatch or crisscross pattern across the top.

Place the cake in the preheated oven and bake for 45 to 50 minutes, or until the top is a deep golden brown and a toothpick inserted in the center comes out clean. Let the cake cool in the pan for 10–15 minutes before transferring to a wire rack to cool completely.