Preheat the oven to 375 degrees F. Line baking sheets with parchment paper.: The kitchen will fill with a faint warm scent as the oven climbs toward 375 degrees F , and that rising heat prepares the metal so cookies set immediately on contact. I always line sheets with parchment paper to prevent sticking and to promote even browning. A common mistake is putting cold dough on a cold pan, which can cause excess spread, so pre-heated pans help achieve consistent edges. Listen for the faint click of the oven thermostat cycling as the temp stabilizes.
In a small bowl, whisk together the flour and baking powder.: When you combine all purpose flour and baking powder , you should see a pale, sandy texture with no visible clumps, and a faint dusty aroma of flour. Whisking aerates this mix, which promotes an even lift in the cookies. If you skip whisking, pockets of baking powder can cause uneven rise. I also sniff for stale flour as a quick quality check; fresh flour should smell neutral and not musty.
In a large bowl, use an electric mixer to combine the butter with the brown sugar and white sugar until creamy. Add the peanut butter, egg and vanilla and continue to mix for 1 to 2 minutes. Add the dry ingredients a little at a time until well-combined. Cover the bowl and refrigerate the dough for at least 30 minutes.: As you beat the butter and sugars, watch the mixture transform from grainy to pale and creamy, with a light, slightly aerated texture that smells buttery and sweet. Folding in peanut butter gives the batter a glossy, nutty sheen, while the egg helps it become cohesive. Adding the dry mix gradually prevents a floury dust cloud and ensures the dough is uniformly hydrated. Chilling firms the dough, improving shape retention during baking and concentrating flavors. A common pitfall is underchilling, which yields flat cookies, or overchilling, which makes scooping difficult. While chilling, I usually clean my workspace and prepare the baking sheets.
Use a cookie scoop to scoop chilled dough onto a baking sheet. Bake 9 to 11 minutes, until the cookies are lightly browned around the edges. Let the cookies sit on the baking sheet for a few minutes, and then move the cookies to a cooling rack to cool completely.: When you portion dough with a scoop, you get uniformly sized cookies that bake evenly and present nicely, and the cooled dough will feel slightly firm to the touch. As the cookies bake, they will expand and the edges will take on a soft golden tint while the centers remain paler and tender. Remove them when the edges show light color, since carryover heat on the sheet finishes the centers without overbaking. Leaving them on the hot sheet for a couple of minutes helps set the base, avoiding breakage when transferred. A frequent error is baking too long; that robs chew and yields a dry interior.
Heat the chocolate in a glass bowl in the microwave in small bursts (stirring along the way) until the chocolate is melted and smooth. Slide wax paper underneath the cooling rack. Add the melted chocolate to a piping bag or a sandwich-sized zip baggie. Snip the corner. Drizzle chocolate on the tops of the cookies in a decorative fashion. Refrigerate the cookies or let them sit until the chocolate is set and has hardened.: Melting milk chocolate chips gradually in short microwave bursts results in a glossy, silky chocolate that smells sweet and milky, and stirs to a smooth ribbon that drips with sheen. Placing wax paper under the rack catches any drips and keeps your counter tidy. When you drizzle, aim for fluid, confident motions to create delicate lines, and watch the chocolate cool into a shiny shell that adds both texture and visual appeal. Chill briefly to firm the chocolate if you want a clean snap, but be careful not to condense moisture on the surface. A common misstep is overheating the chocolate, which can seize and become grainy; if that happens, a small amount of neutral oil can sometimes bring it back together.