You can substitute bittersweet chocolate for half of the semisweet chocolate chips.
Heat oven to 375 degrees. Line several baking sheets with parchment paper, and set aside.
Combine butter and both sugars in the bowl of an electric mixer fitted with the paddle attachment, and beat until light and fluffy. Add vanilla, and mix to combine. Add egg, and continue beating until well combined.
In a medium bowl, whisk together the flour, baking soda, and salt. Slowly add the dry ingredients to the butter mixture. Mix on low speed until just combined. Stir in chocolate chips.
Scoop out 2 tablespoons of dough, and place on a prepared baking sheet. Repeat with remaining dough, placing scoops 3 inches apart. Bake until just brown around the edges, 16 to 18 minutes, rotating the pans between the oven shelves halfway through baking. Remove from the oven, and let cool slightly before removing cookies from the baking sheets.
Store in an airtight container at room temperature for up to 1 week.