Prep potatoes. Peel and thinly slice into rounds. Thickness depends on you! There's no right or wrong here however the thicker they are the more they will need to be fried.
Using a one inch heart cookie cutter, cut as many heart shapes as you can. This will depend entirely on how big your potato is.
Warm up a medium sized skillet over medium heat. Pour in enough oil to fill skillet diameter, and to come up exactly to the height of the potatoes. You really don't need more oil than that. In batches fry potatoes, cooking them 4-5 minutes per side or until golden.
Remove from heat and rest on a paper towel lined plate. You can do all of this the night before! Just make sure potatoes are cool and refrigerate.
When ready to serve, warm up a large skillet. Add in butter. When melted, add in rosemary and stir until fragrant. Toss in potatoes and coat well. Season with salt and serve