You can fill your chocolates with many different things. You could go with ganache, either flavored or plain; some sort of fruit filling, such as raspberry jam; or maybe some kind of creme.
Ganache is super easy to make and it can be flavored any way you want. Ganache is a chocolate a mixture if chocolate and heavy cream, and sometimes butter, that can then be flavored. It is quite often used to coat cakes or make truffles. This is a link to a great explanation of ganache.
A good rule of thumb for ganache is to go with two parts chocolate to one part heavy cream. I generally prefer to scald the cream and pour that over the chocolate, stirring until everything is mixed together. Once the mixing is done you can add a flavoring: some kind of liqueur is your best bet, though you can use fruit puree. Another great way to flavor ganache is to steep something in the cream while you're heating it, such as a flavorful tea or lavender.
You could use peanut butter as a filling, as well as fruit jams, or maybe marshmallow fluff if your sweetie is into that. You could also try some other kind of candy filling. Recipes for candy fillings are all over the internet. This site seemed to have a good selection of tasty choices.
It's a good idea to have you filling made before you are ready to start. It will need to be cooled at the very least. You could even have it ready to be piped into the mold before you start tempering the chocolate (this is discussed in length later). If you don't have store bought cloth or plastic bags, or parchment paper folded into a cone, then you can use plastic baggies. Fill the baggy and snip off one corner for a great home answer to a piping bag.
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