Cover a medium-sized baking sheet completely with aluminum foil, and grease it lightly with cooking spray.
Melt butter in a large saucepan over low heat, but do not let it boil.
Add sugar and increase heat to medium, continuously and gently stirring until a candy thermometer reaches 260ºF.
Add almonds and continue stirring constantly until the candy thermometer reaches 295ºF.
Pour sugar and almond mixture onto the prepared baking sheet, ensuring that it spreads out evenly over the whole sheet pan, including into the corners.
Melt chocolate in the microwave in 15 second intervals or on the stove over low heat until it is spreadable.
When the candy has cooled slightly, spread the chocolate on top, taking care to cover the entire surface and smooth it out.
While the chocolate is still warm, sprinkle the chopped walnuts or pecans over the surface.
Allow to cool completely for several hours in a cool, dry place, or in the refrigerator. Once the candy has hardened, you can break it apart using a mezzaluna or the sharp point of a pairing knife. The candy should automatically shatter into bite sized pieces.
Store candy in an air-tight container at ambient temperature for up to 4 days, or refrigerated for up to a week (not that it will take you that long to eat it).