Preheat the oven to 350F. In a large bowl, beat the butter, peanut butter and brown sugar until creamy; beat in the egg and vanilla. Add the flour, oats, baking soda and salt and beat on low or stir until the dough comes together. As you stir, add the chocolate, pretzels or chips, and anything else you like.
Drop by the large spoonful (or use a scoop) onto a parchment-lined baking sheet and bake for 12-15 minutes, or until golden around the edges, but still soft in the middle. Transfer to a wire rack to cool. Makes about 1 1/2 dozen cookies.