Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan with butter or cooking spray, then lightly flour it.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt until well combined.

In a large mixing bowl, beat the softened butter and granulated sugar until light and fluffy, about 3-5 minutes.

Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

Gradually add the dry ingredients to the butter mixture, alternating with buttermilk, mixing until just combined.

Gently fold in the lemon juice and lemon zest into the batter.

Pour the batter into the prepared cake pan and bake for 30 to 35 minutes until a toothpick comes out clean.

Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

In a saucepan, whisk together heavy cream, granulated sugar, lemon juice, egg yolks, cornstarch, and a pinch of salt. Heat until thickened.

Stir in the butter and lemon zest until smooth and creamy.

Once cooled, slice the cake horizontally into two layers.

Spread a layer of custard on the bottom layer, then place the second layer on top.

For the glaze, mix powdered sugar and lemon juice until drizzleable, adjusting consistency as needed.

Drizzle the glaze over the cake and optionally sprinkle with lemon zest before serving.