Adjust rack to lower third of oven and preheat oven to 400 degrees F. Line a 12-x-15½-x-½-inch baking sheet with aluminum foil, leaving a 2-inch overhang at each short end.
Batter: Pour the cream, corn syrup, and sugar into a 1½-quart heavy-bottomed saucepan. Stir to combine, then add the butter. Stir over medium-low heat until the butter melts. Then raise the heat to medium, and stir constantly with a wooden spoon—especially toward the end of cooking—to prevent scorching. Cook until the mixture reaches 238 degrees F, the soft-ball stage, on a candy thermometer, about 10 minutes; the mixture will appear between ivory-and straw-colored. Remove from heat, and stir in almonds.
Pour the mixture onto the center of the foil-lined baking sheet, and spread it in as thin a layer as possible. A pool about 8 inches in diameter is fine. Bake for 3 minutes; then spread the liquid mixture more evenly over the baking sheet. If the mixture still does not completely cover the baking sheet, bake another minute, then tilt the baking sheet to allow the mixture to spread evenly. Bake for about 5 to 6 minutes total, or until golden but not dark brown. Though the mixture bubbles while baking, the bubbles form more slowly toward the end of the baking time than at the beginning.
Place baking sheet on a wire rack and cool for 2 to 3 minutes. While the mixture is still warm but no longer liquid, cut 1½-inch squares using a ruler and pastry wheel. If the mixture is too warm, it will not cut cleanly. If it is too cool, it will splinter or not cut; in that case return it to the oven for a few seconds to soften it a bit.
Decoration: When the squares are completely cool, place the chocolate in a bowl over water at about 120 degrees F. When about two-thirds of the chocolate has melted, remove the bowl from the water. With a dry rubber spatula, stir until the liquid chocolate melts the soft or unmelted chocolate that remains. Fill a small handmade paper cone with the chocolate and snip off the tip. Pipe thin lines of chocolate back and forth over the squares’ surface. Refrigerate the squares briefly, just to set the chocolate. Remove from the refrigerator and set aside in a cool, dry place.
To store, remove squares from the foil and place them in layers, separated by strips of aluminum foil, in an airtight metal or plastic container in a cool place up to 1 week.