Melt butter in a large pot on medium-low heat. Once the butter is fully melted, you'll add 1 1/2 bags of mini marshmallows into the pot and set the other marshmallows aside for later!
Mix the butter and marshmallows together with a rubber spatula, stirring continuously until the marshmallows have completely melted, making sure to keep the heat on low.
Once the marshmallow mixture is totally melted, remove the pot from the heat and stir in vanilla and salt.
Add roughly half of the rice krispy to the marshmallow mixture, making sure to scrape the bottom thoroughly to incorporate all the marshmallow mixture.
Once that's all mixed together, add in the remaining marshmallows and rice cereal.
Pour the rice krispies into the pan, then gently spread the treat evenly onto the pan.
With parchment paper, wax paper, or the back of a rubber spatula, gently press the rice krispies until you have a flat layer that is slightly thinner than the cookie cutter.
Allow the rice krispy treats to cool for 10 minutes to harden slightly.
Remove the rice krispy treats onto a flat surface using the parchment paper.
Lightly spray the inside of your cookie cutter with a non-stick cooking spray, press it gently into the rice krispy treats, and press out the desired shape. Transfer the finished treat to another tray to allow it to set completely.
Repeat the process, reapplying the non-stick spray as necessary until you run out of room.
To decorate the hearts, place each color of candy melts in a separate microwave-safe bowl. One color at a time, microwave for 30 seconds, stir, then repeat until completely melted and smooth. 1/2 - 1 tsp coconut oil may be added to chocolate if needed to make it smooth and easy to spread.
Use a piping bag, or plastic baggie with the corner tip cut off, to pipe melted chocolate around the edge of heart shaped treats. Then fill in the heart with melted chocolate and spread to the outline with a knife, making it nice and smooth. Repeat with each color until you have finished all the treats.
Once coating is completely cooled, use an edible pen to write sayings on the hearts. Use a damp paper towel to wipe away any mistakes. (Or you can skip this step and free-hand the sayings!)
Choose the color you want the sayings to be and melt the chocolate as noted above. Then use a piping bag with a small hole to write over the edible marker sayings or free-hand sayings on coating.
Notes
Store in an airtight container for up to three days. Longer than that and they'll start to get hard, so it'd be easier to just whip up a new batch!Work in color batches. Choose two colors of candy melts to melt at a time and use those colors to add color to the hearts and to write on the hearts. For example, melt blue and purple at the same time. Then do some hearts with a purple layer and blue writing and some hearts with blue base layer and purple writing. That way you don't have to deal with a bunch of colors all at once.Keep your temperature low on the marshmallow mixture, even if you are tempted to heat it up higher to melt it quicker. Slow and low is the way to go!Make sure to be gentle, you don't want to smash all the cereal pieces.If shaping with a cookie cutter, arrange your cookie cutter to get as many cutouts as possible. You will not be able to reform excess treats to get another cut like if you were making cut out sugar cookies.Avoid using detailed cookie cutters so the shapes come out. You can always add details back in with the decorations.This recipe can also be shaped with a knife by letting it cool on a pan and then cutting out the desired shape or done by hand as described in this shaped rice krispy treats recipe.