Line a small baking sheet or plate with parchment paper.

Add the butter, sugar, water and salt to a small heavy-bottomed saucepan.

Cook over medium heat, stirring as the butter melts and the ingredients combine.

Continue cooking until the mixture reaches 300°F on a candy thermometer and turns a deep golden amber color.

Remove from the heat and carefully stir in the vanilla.

Immediately pour the hot toffee onto the parchment and spread it into a thin layer.

Let cool completely until hard, about 20–30 minutes.

Break the toffee into pieces, then roughly chop it. You want a combination of little bits and larger chunks.

Make the Brown Butter Rice Krispie Treats

Line an 8x8-inch baking pan with parchment paper, leaving some overhang on the sides for easy removal. Lightly grease the parchment.

Add the butter to a large, light-colored pot or saucepan over medium heat.

Melt the butter, then continue cooking while stirring frequently. It will foam and bubble before the milk solids begin turning golden brown. Once you see deep golden specks at the bottom and the butter smells toasted and nutty, remove it from the heat.

Immediately add about 18 ounces of the mini marshmallows, reserving approximately 2 ounces for later. Stir until completely melted and smooth. If necessary, return the pot to low heat briefly to finish melting them.

Stir in the vanilla and salt.

Add the crispy rice cereal and gently fold until evenly coated.

Let the mixture cool for a minute or two, then fold in 1 cup chopped English toffee and the reserved mini marshmallows. Don't overmix—you want pockets of marshmallow and noticeable pieces of toffee throughout.

Transfer the mixture to the prepared 8x8-inch pan. Using lightly buttered hands or a piece of parchment paper, gently press the mixture into an even layer. Don't compact it too firmly or the treats will lose their soft, gooey texture.

Sprinkle additional chopped English toffee over the top, gently pressing the pieces into the surface.