Preheat oven to 300℉. (or 148.88℃.)

Measure flour, baking powder and salt into a medium bowl. Use a whisk to combine; set bowl aside.

In separate large bowl, use electric mixer to beat brown sugar with granulated sugar on MEDIUM speed until combined. Add soft butter; beat until fully incorporated and mixture has a texture similar to wet sand. Add egg, sour cream and vanilla. Beat on medium speed until fully incorporated and batter is fluffy.

Scrape batter down from the sides of the bowl. Add reserved flour mixture; beat on LOW speed to incorporate flour only until batter is thick and just barely combined. Be careful not to OVER-MIX the batter.

Melt chocolate-it's IMPORTANT to not get water or moisture in the chocolate or it will "seize up" and get hard; Here's how to melt chocolate chips in a microwave. Place chocolate chips in a dry microwave-safe container. Heat on HIGH for 30 seconds. Remove bowl; stir. Repeat in 30 second pops of power (HIGH heat) and remove after each one to stir chocolate. Important- Only melt it in 30 second bursts so chocolate doesn't burn. It takes about 4-5 rounds of 30 seconds of heating/stirring to fully melt chocolate.

Let fully melted chocolate cool for 1 minute, then pour it onto the dough. Use a wooden spoon or spatula to swirl chocolate through the batter. Do NOT completely mix in the chocolate, simply swirl it through for a nice, marbled look.

Drop dough by Tablespoons 2" apart onto UNGREASED cookie sheet(s). Bake at 300°F. for 20-22 minutes. When done, cookies will be "set" around edges and light brown in appearance. Place baking sheets on a wire rack; let soft cookies cool 2 minutes (to firm up slightly) before removing them from the pan.

Use a spatula to transfer cookies from baking sheet onto a wire rack to finish cooling. Serve at room temperature and enjoy!