Specular Map not Baking Correctly

I'm trying to bake a specular map onto my low poly model. I created a black shader and applied it to my high poly model with a direct light and a front facing camera in the scene.

I want to bake this material with the lighting detail onto my low poly, however, the map always comes out with none of the material or the lighting detail (I've shown the UV map below).

Not sure if I'm missing something obvious. I'm new to Marmoset and still learning about textures / baking this way so any help would be much appreciated.

(Edit): For a bit more context, I'm trying to achieve the same effect as in one of the videos in Yekaterina Bourykina's handpainted tutorial (https://www.artstation.com/learning/courses/Ggz/baking-maps-and-blocking-in-key-elements/chapters/E9E/bakes - around 2:45 in)