Line a 15x10x1" jelly-roll pan with non-stick foil,or spray regular foil with non-stick spray. In medium saucepan; heat butter, sugar, corn syrup, and 2 tbsp water. Heat over medium heat to melt butter. Increase heat to med-high and insert candy thermometer.
Cook mixture about 12 minutes until golden brown and temp reaches 300°F DO NOT STIR. Remove from heat and remove thermometer. Carefully stir in 1c. chopped almonds.
Quickly pour toffee onto prepared pan and spread to edges. Let stand in pan on a wire rack for 30mins.
Melt half of the chocolate in microwave on High for 45 seconds. Stir to smooth. Spread over top of cooled toffee. Refrigerate 20 minutes until firm.
Once bar is firm, flip over onto a piece of wax paper; remove foil. Microwave remaining chocolate on High 45 seconds and stir to smooth. Spread over toffee bar; sprinkle with remaining 1/4c. almonds. Use wax paper to slide toffee bar back onto baking pan. Refrigerate 20 more minutes.
Once chocolate is firm, break bar into 2.5" pieces. Store airtight at room temperature up to 1 month.