Preheat oven to 350°F. Remove butter from the fridge and let it warm to room temperature. Line an 8x8-inch square pan with parchment paper or grease well. (I recommend parchment because you can lift it out to neatly cut the squares later.)

In a medium saucepan, over medium heat, simmer raisins with enough water to cover. Stir frequently and add water as necessary until raisins plump (20-30 minutes). Stir in brown sugar and salt. Make sure cooking liquid (which will be a little syrupy by this time) comes to the top of the raisins. Dissolve custard powder in a couple tablespoons of cold water
and stir in with the raisins; return to a simmer to cook out the starch and thicken up to the consistency of pie filling. Stir in vanilla; set aside to cool.

In a large bowl, cream together butter and brown sugar. Add egg and van-illa. In another bowl, whisk together flour, baking powder and salt. Add to creamed mixture and mix just until it comes together as a soft dough. Trace the bottom of your baking pan onto a sheet of parchment paper. (If you don’t have parchment, trace the shape onto a sheet of looseleaf and place a sheet of plastic wrap on top so you can see the guide through the plastic.) Roll out half the dough to 1/3-inch thick and to the shape of the pan you traced, trimming as necessary. Invert this into the baking pan and press into place. It should lie flat and not come up the sides. Trim dough if necessary. Repeat the process with the remaining dough to form the top crust. Refrigerate the top and bottom crusts until you are ready to add the filling.
To assemble, remove parchment paper from crusts. Pour cooled filling over the bottom crust; smooth flat. Invert the top crust over the filling. Press lightly to fit. Bake at 350°F for 25-30 minutes until golden brown. The squares must cool completely in the fridge for several hours before you cut them or else the filling will ooze out and the pastry will crumble.

With a sharp, thin knife, wiping between cuts, cut into 16 squares (four columns horizontally and vertically). Or for dessert-sized portions, cut 3x3 or 3x4 columns, for nine or 12 portions respectively. These freeze well as long as they’re tightly wrapped so as to not dry out.