Heat 1 tablespoon butter in a skillet over medium-high heat.
Season the chicken breasts with salt and pepper, then sear in the hot skillet until golden brown, about 3 minutes per side.
Place the seared chicken breasts in the 6-quart slow cooker.
Cut the remaining 4 tablespoons of butter into pieces and place one piece on top of each chicken breast.
Add the dried basil, crushed red pepper flakes, lemon juice, and lemon zest to the slow cooker.
Cover and cook on LOW for 3 hours 30 minutes (or on HIGH for just under 2 hours).
In a small bowl, whisk 1 tablespoon of the half-and-half with the cornstarch until smooth, then whisk that mixture into the remaining half-and-half. Stir in the chicken base and the rinsed capers.
Pour the half-and-half mixture into the slow cooker, stir to combine, then cover and cook on HIGH for 1 more hour.
Check seasoning and serve the creamy lemon chicken warm.
