Set rack in the middle of the oven, and heat oven to 350˚F. Line an quarter sheet pan with parchment paper, leaving some flaps on each side. Set aside.
If you'd prefer to use large marshmallows, cut about 24 of them into thirds or halves and reserve. Obviously, thicker pieces make a thicker marshmallow layer: please yourself in this.
SHORTBREAD BASE
Using an electric mixer, beat the butter and sugar together until pale and fluffy. Stop the mixer, scrape down the bowl, then add in vanilla paste (or extract) and egg. Beat again, until well mixed. Stop mixer again, add flour and salt, and beat on low speed until completely incorporated.
Scrape batter into prepared pan. With clean wet fingers (wet fingers keeps the dough from sticking to them) press the dough across the bottom of the pan. Work quickly; try to make the dough as even as you can.
Bake crust 30-35 min, until golden brown around the edges but still puffy and soft in the center. Remove crust from the oven and let rest about 20 min.
FILLING BASE LAYER
If you will use chocolate, sprinkle the crust with some good quality chocolate chips, and return to the oven for 3-4 minutes. Remove from oven and spread the chocolate evenly and gently. A small offset spatula is the perfect tool for this. Let the crust rest about 10 more minutes.
Substitutes for chocolate in this layer: Nutella, butterscotch chips (melt as above), peanut butter, almond butter.
FILLING JAM LAYER
Add a layer of fruit preserves. I had some black raspberry jam to use. Apricot or raspberry or cherry would be great with chocolate – this is where your imagination comes in. At any rate, you’ll use about ¾c of the preserves, and spread them thinly.
TOP WITH MARSHMALLOW
Now arrange the sliced marshmallows on top of the jam. Return the pan to the oven for 4-5 minutes, to soften the marshmallows. Remove from the oven, and turn the oven to use the broiler setting.
Use the back of a spoon (or your trusty offset spatula) to gently press and smoosh the top of each marshmallow. Once they are all smashed, return the pan to the broiler. Watch the tops carefully! Toast the marshmallows to your desired shade; remember that they brown very quickly.
Remove the toasted cookie bars from the oven. You’re finally finished! Allow to rest at least 10 minutes, then lift the parchment right off the quarter-sheet pan onto a cutting board. Use a wet knife to slice into 24 bars, wiping the blade with a wet cloth after each slice.¾
STORE IN AIRTIGHT TIN
The cookies will keep, well wrapped, at room temperature for up to 4 days. However, today, within three hours, the pan was mostly devoured by the three of us. You have been warned!
Notes
NOTE: That flour measurement is tricky! Scant 1 3/4 cups flour sounds so much better than 1 11/16 cups, doesn't it!