In a medium bowl with a hand mixer, mix butter, sugar, and Sizzle until well combined. This can be done with a whisk, but may require a little extra elbow grease! 

Add egg yolk and vanilla and mix again until combined. Switch to a rubber spatula, add flour and salt, and stir to combine. 

Separate ¾ of the dough onto a clean work surface. To the remaining dough, add gochujang and stir to combine. (Optional: if you want the color to really pop, add a few dabs of red food coloring.)

Place the gochujang dough on top of the plain dough, and with you hands, mash and fold the dough together a few times. You'll start to see the gochujang dough marble into the plain dough. A good test to see if you've mixed enough is to separate the dough in half and see what the cross-section looks like. If you've got a few streaks of gochujang dough ruining through plain dough, you're all set! 

Divide the dough in half, form each piece into a log, wrap each log in plastic wrap or parchment paper, and freeze for about 1 hour, just until it's firm enough to slice. 

When you're just about ready to start slicing the cookies, preheat the oven to 350F and line your cookie sheets with parchment paper. 

Slice the cookies into ¼ inch slices and place them on your tray about 1 inch apart. Pop them back into the freezer for 10 minutes- this will help them hold their shape better. 

Remove the cookies from the freezer and bake for 9-10 minutes, just until the edges look a little golden brown. Remove from the oven and let cool on the try for 5 minutes before moving to a cooling rack. Let cool completely before enjoying!