Grease two sheet pans with butter.
Combine butter, sugar, and water in a deep pot. Attach a candy thermometer to the side of the pot with the end in the liquid but not touching the bottom of the pot. Cook the mixture over medium heat until it reaches 250°F, about 10 minutes.
Add the sliced almonds and stir to combine. Continue to cook the mixture until it reaches 300°F, then immediately pour it onto the prepared pans, dividing it evenly. Do not move the pans around; let the caramel spread naturally. Let the brittle sit until it is just cool enough to safely handle it, about 5 minutes; it should have hardened slightly around the edges and shouldn’t be goopy in the center—if it is, wait another minute or two. Gently flip each big piece of brittle over, set it back into the same pan, and use a paper towel to very carefully wipe the butter off the top of the brittle, trying not to break it.
While the brittle continues to cool, prepare the chocolate: Melt the chocolate chips in a double boiler (or in a heatproof bowl set over a pot of simmering water) over medium heat, stirring frequently with a rubber spatula.
When the chocolate has melted, spread one-fourth of it onto each slab of brittle and sprinkle each with one-fourth of the crushed almonds. Flip both slabs over, placing them back in their pans (it’s fine if some of the almonds fall off). Divide the remaining melted chocolate between the two slabs. Spread it out, and sprinkle it with the remaining almonds. Let the chocolate firm up until it has fully hardened, at least 2 hours, then break the brittle into smaller pieces to serve or wrap up. Store it in an airtight container so that it maintains its crisp texture.