Place your softened butter in a bowl or stand mixer. Cream the butter using the stand mixer with a whisk attachment or hand mixer until smooth and fluffy, about 5-7 minutes. Scrape down the sides as needed.
Whisk and sift together the flour and sugar. Add it to the butter 2 tbsp to ¼ cup at a time, beating it with the stand mixer or hand mixer until fully incorporated before adding more. Scrape down the sides as needed.
Once you near the end of the flour/sugar mixture (about ¼ to ½ cup left), add just a bit at a time and mix by hand until a dough forms that stays together without sticking to your hands. You'll probably have about ¼ left when the dough has formed.
Place a sheet of parchment paper on a surface and dust with the leftover flour/sugar mixture (or just flour if there was none left). Place the dough ball and dust with a bit more of the mixture. Place another sheet of parchment paper on top and then roll out the dough until it is a ¼ inch thick with a rolling pin.
Place the rolled out dough in the fridge for 20 minutes.
Preheat the oven to 350°F. Line a baking tray with a sheet of parchment paper.
After the dough has stiffened a bit, remove it from the fridge and cut circles using a 2 inch across cookie cutter. Place the circles on the baking tray.
Reroll the extra pieces of dough and repeat.
Once all of your circles have been cut, spoon about 1 teaspoon of your filling into the center of each circle.
Fold up three sides of each circle and pinch the sides the touch together to create the triangle shape. Be sure to seal/pinch the edges well so that they don't fall apart or leak when baking.
Place the baking tray in the freezer for 20 minutes.
Remove from the freezer and transfer the baking tray to the oven. Bake for 15 minutes, or until the bottom edges of the cookies start looking golden brown.
Remove and immediately transfer to a wire rack to cool completely.