Line an 8 × 8-inch (20 x20 cm) square pan with parchment paper, leaving some overhang for easy removal.
Place the raisins in a bowl and cover with warm water. Let soak for about 5 minutes, just until softened. If your raisins are very fresh, soft, and plump, you can skip this step. Drain well and lightly pat dry. They do not need to be completely dry.
Add the raisins to a food processor and blend until they form a thick paste. Add the almond butter, cocoa powder, and vanilla, if using, and process until fully combined. The mixture may look slightly crumbly, but it should hold together easily when pinched. If the mixture feels too dry, add 1 to 2 TBSP of water and blend again, just until it comes together.
Transfer the mixture to the prepared pan and press it down firmly and evenly, making sure it’s well compacted.
To avoid burning the chocolate when melting, melt the chocolate chips in the microwave in 30-second intervals, stirring between each, until fully melted and smooth. Pour the melted chocolate over the base layer in the pan, spreading it evenly. Scatter the chopped walnuts on top and finish with a pinch of flaky sea salt.
Refrigerate for about 30 minutes, or until the chocolate is set. Lift out of the pan, cut into bars, and serve.