1. Preheat the oven to 350 degrees F. Grease a 9-inch layer cake pan. Line with a circle of parchment paper, if you have it.
2. In the bowl of an electric mixer, beat the butter with the sugar until fluffy and pale. Add the salt, then add the eggs one at a time, beating well after each addition. With the mixer on low speed, add the flour and enough milk to so that the batter is not too stiff.
3. Pour the batter into the prepared pan and bake in the oven for 25 minutes, or until it’s golden brown and a toothpick inserted in the center comes out clean. Allow to cool for a few minutes before removing the cake from the pan (peel away the parchment paper if you used it).
4. Let the cake cool completely, then use a long, sharp knife to slice through the center so that you have two layers that are approximately the same height (don’t worry if they're not exactly even, though). Spread the lemon curd (or jam) onto the bottom layer, then put the other layer on top. Dust with powdered sugar, or mix remaining lemon curd with mascarpone and use this to thinly ice the cake. This cake will keep well in an airtight container for 4-5 days: common sense tells me I should have refrigerated the leftovers because of the eggs in the lemon curd, but I didn't...you do what feels right to you.