Melt the butter in a medium saucepan over medium heat. Add the onion and cook, stirring occasionally, for 5 to 8 minutes, until tender and translucent.

Add the brown sugar and continue cooking for another 8 to 10 minutes, until caramelized.

Add the garlic, chili powder, salt, and pepper and cook over medium heat for 1 minute.

Remove from the heat, place into a bowl, and set aside until room temperature. (You can speed this up by popping it in the refrigerator.)

In a large bowl, combine the pork, beef, egg, bread crumbs, vinegar, Worcestershire sauce, and cooled onion mixture and mix just until combined. Don’t overmix.

Shape into 8 patties for ¼-pound burgers.

Place on a baking sheet, cover with plastic wrap, and refrigerate for at least 2 hours or up to 2 days. You can also place in the freezer for 30 to 60 minutes. (This helps the burgers to hold together!)

In a skillet on the stovetop or on the grill, cook the burgers for 4 to 5 minutes per side on medium-high heat, until a meat thermometer inserted into a burger reaches 160°F. Serve as desired.