Weigh out the butter while cubing & leave out to soften & come to room temperature. Small cubes (about 1cm, less than ½"), have a smaller surface area & soften faster. Watch this hack for Softening Butter In About 12 Minutes. See Photo 1 below. Set the scales to zero (TARE), with the mixing bowl on top, & weigh in the sugar. Then add the softened butter & beat with a mixer until soft & well incorporated. You will be able to easily spread the mixture, but it will not be completely smooth. See Photo 1 above.
Place the bowl back on the scales, set to zero & weigh in the flour, salt & baking powder. Mix until all is incorporated and no flour is visible. The mixture will appear dry, but if squashed together, it comes together. See Photo 2 above. Finally weigh in the oats & fold in until it is all incorporated. See Photo 3 below.
Turn the oven on to heat up to: 170°c/150°c Fan Oven/325°f/Gas Mark 3, while you prepare your baking tin.
I grease the tin (with baking or cooking spray like Fry Light, or home made lining paste) and then cut out baking/parchment paper to fit the tin. Place enough of the baking paper under the baking tin that it will cover up the sides of the tin and then cut out the corners, (you can draw where to cut or do free-hand). Then simply press the paper into position in the tin, adding more grease if required. See Photo 4 above. Split the mixture in half & drop small pieces into the bottom of the prepared tin. Then with slightly wet hands, spread the mixture down & out in order to completely cover the base of the tin. Then flatten slightly with the palms of your hands. See Photo 5 above.
Mix the jam/jelly or curd, with a small amount of water in a small bowl until it is thinned slightly and easier to spread, if needed. Spoon the jam/jelly/curd over the mixture & spread right to each corner. Use a spatula or knife to spread it out equally. See Photo 6 above & 7 below.
Now take the remaining half of the mixture & place small pieces of it on top of the jam/jelly/curd layer until you have used it all up. See Photo 8 below. Then spread to cover the middle layers, as best as possible. Using wet hands press down gently and even it out. Don’t worry if there are any jam/jelly pieces visible – these will cook up lovely. See photo below.
Place in the centre of the oven for 15 mins. After this time, turn the tin around and continue bake for another 15 mins. (See Photo 9 below). To check if it’s done, gently shake the tin & if the contents still wobble, it needs more time. If not ready, bake for a further 2 mins. Check again for wobble. If it still wobbles, turn the tin and bake another 2 mins.
Once there’s no wobble, you will try the centre with a clean cocktail stick. You are not looking for a clean stick when pulling it out, but rather that you have made a reasonably sharp hole in the middle. Remove from the oven and place on a cooling rack (while still in the tin) and leave to cool completely. See Photo 9 above.
Once cooled enough, lift out from the tin using the paper and lay on a chopping board. Cut into 4 strips and then turn and do the same again to produce 16 squares. See Photo 10 above and Photo 11 below of the texture once set and cut.
Store in an airtight container, preferably in a tin, with the paper still under it to help keep in the moisture.