Bake the Sponge
Preheat oven to 350°F (175°C). Grease and line two 8-inch cake pans with parchment.
Cream butter and sugar together until pale and fluffy. Add eggs one at a time, beating well. Stir in vanilla, lemon zest, and lemon juice.
In a separate bowl, whisk flour, baking powder, and salt. Alternate adding the dry mixture and milk into the wet ingredients, mixing gently.
Divide batter between the pans and bake for 20–25 minutes, or until golden and springy. Cool completely on a wire rack.

Make the Lemon Custard
In a medium saucepan, whisk together milk, sugar, egg yolks, cornstarch, lemon zest, lemon juice, and salt. Cook over medium heat, stirring constantly, until thickened and smooth (about 6–8 minutes).
Remove from heat and stir in butter. Transfer to a bowl, press plastic wrap directly on the surface, and refrigerate until completely chilled.

Assemble the Cake
Place one sponge layer on a cake stand or serving plate. Spread the chilled lemon custard evenly over the top. Gently place the second cake layer over the custard.

Decorate & Serve
Dust the top with powdered sugar. If desired, add a swirl of whipped cream and a fresh mint leaf in the center. Chill for 15–30 minutes before slicing for clean edges.