Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

In a large bowl, whisk together flour, baking powder, and salt.

In a separate bowl, cream together butter and sugar until light and fluffy. Beat in eggs one at a time, then stir in vanilla and lemon zest.

Gradually add dry ingredients to wet ingredients, alternating with milk, beginning and ending with dry ingredients. Beat until just combined.

Pour batter into prepared pans and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Let cakes cool in pans for 10 minutes before inverting onto a wire rack to cool completely.

In a saucepan, whisk together sugar, cornstarch, and salt. Whisk in lemon juice and egg yolks.

Cook over medium heat, whisking constantly, until the mixture thickens and bubbles. Remove from heat and whisk in butter until melted.

Pour filling into a bowl, cover with plastic wrap (pressing it directly onto the surface to prevent a skin from forming), and refrigerate until chilled.

In a chilled bowl, whip heavy cream, powdered sugar, and vanilla extract until stiff peaks form.

Place one cake layer on a serving plate. Spread with lemon cream filling. Top with second cake layer.

Frost the top and sides of the cake with whipped cream frosting.

Garnish with lemon slices or zest, if desired. Refrigerate until ready to serve.