Heat oven to 350 and line an 8x8 pan with parchment paper and spray with cooking spray.
In a stand mixer, cream the butter and sugar on medium high until light and fluffy. Add in the vanilla and yolks, and mix again on medium until well combined, scraping the bowl to make sure everything is well mixed.
In another bowl, whisk together the flour, baking powder and salt. Add this to the mixer and mix just until combined. Hand mix the last bit to be sure it is combined.
Spread 1/2 the batter in the bottom of the pan. I used cooking spray on clean hands to do this as the dough is sticky. Microwave the marmalade for 20 seconds to loosen it up and stir it. Spread it over the dough. Drop the chocolate around over this and then drop spoonfuls of the rest of the dough over the top and again, spread it with greased hands. You don't need it to full cover as you actually want some marmalade and chocolate showing through.
Bake for 15 minutes, then rotate and bake another 15 minutes or so until the top is golden brown and it is set in the middle. Cool for 30 minutes, then remove from the pan and cool fully before cutting into squares. These can be stored in an airtight container at room temp, and they also freeze well.