Soften butter. Bring butter to room temperature by leaving it out overnight or for several hours. You should be able to easily press a finger into it, but it shouldn't be melted or greasy.NOTE: If using homemade butter, proceed from here directly after rinsing but before chilling the butter.
Roast garlic. Cut off the top of a bulb of garlic, place it in a ramekin or the center of a sheet of aluminum foil. Drizzle with olive oil, salt, and pepper and roast in the oven for 30-45 minutes until golden brown and fragrant. The cloves should shrink slightly in their papery wrappers.When cool enough to handle, use a fork or paring knife to remove the cloves from their papery wrappers. Mash the garlic cloves with a fork and set aside until ready to use.
Combine softened butter, roasted garlic cloves, fresh herbs, and salt (if using) in a bowl or using the paddle attachment on your stand mixer. Mix until very well combined.
Cut a sheet of parchment paper about 10" long.
Transfer the softened compound butter to the center of the parchment paper in a long, rough log shape.
Fold the top of the parchment paper forward over the log of butter. Place your hand on the bottom sheet of parchment paper near you to anchor it in place. Then press a bench scraper at an angle into the top layer of parchment paper where the butter log meets the counter. This will create tension in the parchment paper and force the butter into a long smooth log shape. Repeat until the log is as long as you want.
Wrap any remaining parchment paper around the log of butter and place in the fridge to firm up, at least 1 hour. Once the butter is fairly firm, wrap the log well in plastic wrap and store in the fridge until ready to use.
Store compound butter tightly wrapped in the fridge 5-7 days, or in the freezer up to 6 months.