In a food processor, combine the basil leaves, garlic, Parmesan cheese, and pine nuts (if using). Pulse several times until the basil leaves are chopped into tiny pieces (stop and use a spatula to scrape down the basil if needed).
Add the rest of the pesto ingredients (⅓ cup olive oil, lemon zest, 1 tablespoon lemon juice, ¼ teaspoon salt, and pepper) and process until the pesto is as smooth as you like. You can add an extra splash of olive oil if needed. Give it a taste and see if you need to add more salt, lemon juice, or pepper.
Make compound butter:
In a medium bowl, combine a stick of butter and 2-3 tablespoons pesto. Use a fork to mix the pesto in until completely combined and no white chunks of butter remain. Give it a taste, and see if you need to season with salt or want to add more pesto.
Use immediately, or transfer to a glass mason jar and refrigerate. Pesto butter will keep well for up to a week.
Notes
For freshest-tasting pesto butter, use homemade pesto made with freshly grated Parmesan and don't skip the lemon juice!Use pesto butter on:
Garlic bread: spread the garlicky pesto butter on Italian bread and toasting.
Steak: scoop a spoon of pesto butter on during the last minute of cooking.
Use as a spread in a sandwich instead of plain butter or mayo.
Spread on a toasted baguette as an appetizer.
Stir some into roasted potatoes right as they come out of the oven.
Grilled chicken: spoon some on right as the chicken comes off the grill.
Spread some softened pesto butter on a salmon fillet, then bake or air fry. You can also pan-fry salmon fillets in pesto butter.
Elevate your grilled cheese.
Melt some pesto butter in your skillet and use that to cook scrambled eggs.
Spread on warm biscuits or toast for breakfast.
The nutrition facts are estimated assuming 16 servings, or ½ tablespoon of pesto butter.