Gently melt the coconut oil in a microwave-safe bowl or in a small saucepan over low heat.
Whisk in the unsweetened cocoa powder, sugar substitute, chopped walnuts, and tahini paste until the mixture is smooth and fully combined.
Carefully pour the chocolate mixture into 14 cavities of a silicone mold or mini muffin tin.
Refrigerate for about 15-20 minutes, or until the fat bombs are partially set, then gently press a walnut half onto the top of each one.
Return the mold to the refrigerator and chill for at least 30 more minutes, or until completely firm.
Once firm, pop the fat bombs out of the molds. Store them in an airtight container in the refrigerator for up to one week.
Adjust the sweetness to your liking. Start with one tablespoon of sweetener, taste the mixture, and add more if desired before pouring into the molds.