Transfer the roasted peanuts to a food processor or blender. Add the sugar (if using) and salt. Blend on high speed for a few minutes until the peanuts are finely ground and start to clump together.
With the food processor or blender still running, gradually drizzle in the oil. Start with 1 tablespoon and add more if needed to reach your desired consistency. If your peanut butter is too thick, you can add a little water, 1 tablespoon at a time, and continue blending until you achieve the desired consistency.
Stop the blender or food processor and taste the peanut butter. If desired, you can add more sugar or salt to suit your preference or even a pinch of ground cinnamon. Blend again briefly to incorporate any additional ingredients.
Transfer the peanut butter to a clean, airtight jar or container. It can be stored at room temperature for up to two weeks or in the refrigerator for up to three months.