Bake-in chocolate fillings are strange animals because no matter what you do the chocolate is going to seize and go grainy, at least to some extent. Bar chocolate, chocolate sticks, ganache, chocolate chips, chocolate pastry cream…none of them will be the same after all that high oven heat. Which means a bake-in chocolate filling will never be creamy. Assuming you can accept that, and I have a feeling you can, proceed.
You’ll want to use a dark chocolate since that will bring the most chocolate flavor to the party. Start by chopping it as finely as you reasonably can. There’s nothing wrong with a few larger chunks in there.
Next combine the sugar and cinnamon. Why the sugar when chocolate is so delicious by itself? Because fillings are like sauces, they’re meant to add flavor to something larger than themselves. Thus their flavor profile often needs to be exaggerated. But on we go…
Add the butter and stir.
Then pour in the chopped chocolate. Stir thoroughly and it’s ready to use.