Melt the butter and sugar together in a large saucepan over medium-high heat, stirring occasionally until the mixture begins to boil. When the mixture becomes caramel-colored and reaches the “soft crack” stage, 282˚F on a candy thermometer, remove from heat. (Watch carefully as undercooked toffee will be too soft, and overcooked toffee can burn.)
Stir in almonds; pour immediately onto rimmed baking sheet and spread into a thin layer with the back of a spoon.
Break the chocolate into pieces and scatter them on top of the toffee. Let sit for 3 to 4 minutes, so the hot toffee melts the chocolate. Spread the chocolate evenly over the toffee with the back of a spoon; then sprinkle the pecans over the melted chocolate.
Let cool completely and refrigerate until firm, about 2 hours. Break into pieces before serving.
Tip: Before refrigerating the toffee, lightly score the surface with a sharp knife to make it easier to break it into smaller pieces later. If you make more than one batch, consider adding variety by using dark chocolate instead of milk chocolate.