Butter Breton Cake

Instructions

1. Prepare the baking pan
   Preheat your oven to 350°F (175°C). Butter a 9-inch (23cm) round cake pan or a fluted tart pan with a removable bottom. Line the bottom with parchment paper for easy removal.
2. Cream butter and sugar
   In a large bowl, beat the softened butter and sugar together until the mixture is pale and fluffy, about 3-4 minutes. This is a crucial step for the cake's texture.
3. Add egg yolks
   Add the egg yolks one at a time, mixing well after each addition. Then stir in the vanilla extract, salt, lemon zest, and rum (if using).
4. Incorporate the flour
   Gradually add the flour to the butter mixture, folding it in gently with a spatula until just combined. Be careful not to overmix, as this will develop gluten and make the cake tough.
5. Shape the dough
   Transfer the dough to the prepared pan and spread it evenly with a spatula or the back of a spoon. The dough will be quite thick, similar to cookie dough. Smooth the top as much as possible.
6. Create the traditional pattern
   Beat the whole egg with 1 tablespoon of water to make an egg wash. Brush the top of the cake with the egg wash. Using a fork, create a traditional criss-cross pattern on the surface of the cake.
7. Bake the cake
   Bake in the preheated oven for 40-45 minutes, or until the cake is golden brown and a toothpick inserted into the center comes out clean. The cake should be firm to the touch.
8. Cool and serve
   Allow the cake to cool in the pan for 10 minutes, then remove it and transfer to a wire rack to cool completely. Serve at room temperature. The cake will keep well in an airtight container for up to 5 days.