Preheat oven to 350 degrees. Grease and line a 9x9 pan with a parchment sling.
In a large bowl, cream together butter and sugar until light and fluffy. Add in egg and vanilla, mixing well.
In a separate bowl, sift together flour, baking powder and salt. Add to butter mixture and combine.
Press 3/4 of the cookie mixture into the bottom of the prepared pan (it helps to get your hands wet while you're pushing the dough into the pan). Bake for 20 minutes. Remove from oven, but keep oven on.
Spread dollops of jam and lemon curd over the baked dough. Crumble remaining sugar cookie dough over the top of the jam and lemon curd and bake for an additional 25 to 30 minutes.
Remove from oven and cool completely before slicing and serving.