Lightly butter a 17 by 12 by 1-inch baking sheet (half sheet pan), and line with parchment paper to cover the bottom and all sides. (The butter will anchor the parchment to the pan.)

Melt the butter in a large pot over medium-high heat. Add the sugar, water, lemon juice, and corn syrup and and bring the mixture to a gentle boil, stirring to dissolve the sugar. Once the mixture boils, stop stirring. Brush down any sugar crystals clinging to the sides of the pot with a pastry brush dipped in water.

Clip a candy thermometer to the side of the pot and cook for 15 minutes, or until the mixture reaches 300°F. If the mixture threatens to boil over, lower the heat as necessary. (If the liquid is too shallow to measure the temperature, tilt the pot to get an accurate reading.) Resist the temptation to stir; if the caramel is not coloring evenly, swirl the pan from time to time.

Remove the pot from the heat and stir in 2 cups of the almonds. Quickly spread the caramel in an even layer on the prepared baking sheet. Let cool completely.

Spread the chocolate over the top of the caramel, then sprinkle with the remaining 1½ cups almonds. Let the chocolate harden at room temperature or in the refrigerator.

Break the roca into irregular pieces. Store in an airtight container at room temperature or freeze for longer storage.