14 tablespoons unsalted butter (1 3/4 sticks), softened but still cool, about 65 degrees
1 cup granulated sugar
1/4 cup packed light brown sugar
1 large egg
1 teaspoon vanilla extract
2 1/2 cups old-fashioned rolled oats
2 cups rice krispies cereal (optional- if want really really crispy all the way through)
dash of cinnamon (optional)
Adjust oven rack to middle position and heat oven to 350 degrees. Line 3 large (18- by 12-inch) baking sheets with parchment paper/wax paper/silpat. Whisk flour, baking powder, baking soda, and salt (and cinnamon if using) in medium bowl.
In standing mixer fitted with paddle attachment, beat butter and sugars at medium-low speed until just combined, about 20 seconds. Increase speed to medium and continue to beat until light and fluffy, about 1 minute longer. Scrape down bowl with rubber spatula. Add egg and vanilla and beat on medium-low until fully incorporated, about 30 seconds. Scrape down bowl again. With mixer running at low speed, add flour mixture and mix until just incorporated and smooth, 10 seconds. With mixer still running on low, gradually add oats and mix until well incorporated, 20 seconds. Give dough final stir with rubber spatula to ensure that no flour pockets remain and ingredients are evenly distributed. If using rice krispies and/or chocolate chips, stir/fold them in using the spatula making sure they are incorporated into the dough. You’ve got to really press the krispies into the dough to get them incorporated.
Use small cookie scoop or your hands, roll balls about 1 rounded tablespoon. Place cookies on prepared baking sheets, spacing them about 21/2 inches apart. Using fingertips, gently press each dough ball to 3/4-inch thickness to make flatter.
Bake 1 sheet at a time until cookies are deep golden brown, edges are crisp, and centers yield to slight pressure when pressed, 14 to 16 minutes, rotating baking sheet halfway through. Transfer baking sheet to wire rack; cool cookies completely on sheet. Cookies will crisp up as they cool.