Prepare the pan: Line an 8x8-inch square pan with parchment, leaving overhang on two sides for easy lifting. Lightly grease the parchment if you want extra insurance.
Melt the base: In a microwave-safe bowl, combine chocolate, coconut oil, and nut butter. Microwave in 20–30 second bursts, stirring between each, until smooth and glossy.
Alternatively, melt gently in a heatproof bowl over a pot of barely simmering water.
Sweeten and flavor: Whisk in maple syrup or honey, vanilla extract, and fine sea salt while the mixture is still warm. Taste and adjust sweetness if needed.
Add chia seeds: Stir in chia seeds until evenly distributed. Let the mixture sit for 2–3 minutes to thicken slightly.
Optional mix-ins: Fold in any extras like chopped nuts, shredded coconut, or a pinch of espresso powder.
Keep add-ins to about 1/2 cup total to maintain the fudge texture.
Spread and smooth: Pour the mixture into the prepared pan. Smooth the top with a spatula. Sprinkle flaky sea salt if you like.
Chill to set: Refrigerate for 2–3 hours or until firm.
For faster setting, chill in the freezer for about 45–60 minutes.
Slice and serve: Lift the fudge out using the parchment overhang. Warm a sharp knife under hot water, wipe dry, and cut into small squares.