For the lemon curd filling:
3 large egg yolks
½ cup (100 g) granulated sugar
¼ cup (60 ml) fresh lemon juice
1 tbsp lemon zest
4 tbsp (60 g) unsalted butter, cut into pieces

For the frosting:
1 cup (230 g) unsalted butter, softened
4 cups (480 g) powdered sugar
2 tbsp lemon juice
2 tbsp heavy cream (or milk)
1 tsp vanilla extract
Yellow food coloring (optional)

Instructions
1. Make the cake layers:
Preheat oven to 350°F (175°C). Grease and line three 8-inch (20 cm) round cake pans with parchment paper.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, beat butter and sugar together until light and fluffy (about 3 minutes).
Add eggs one at a time, mixing well after each addition. Stir in lemon zest and vanilla extract.
Mix lemon juice with milk. (It may look slightly curdled; this is normal.)
Add dry ingredients to the butter mixture in three parts, alternating with the milk mixture, beginning and ending with dry ingredients. Mix until just combined.
Divide the batter evenly between the 3 pans and smooth the tops. Bake for 20–25 minutes, or until a toothpick inserted into the center comes out clean.
Let cakes cool in the pans for 10 minutes, then remove from pans and cool completely on a wire rack.