Brown Sugar Cake with Toffee Streusel

Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan and set aside.

Combine flour, sugar, brown sugar, baking soda, and 1/2 teaspoon salt in the bowl of an electric mixer. Stir until well blended. Cut in butter with a pastry knife or fork until the mixture resembles coarse meal. Set aside 1/2 cup of the mixture.

Whisk buttermilk, egg, and vanilla extract until the egg is well incorporated. Add buttermilk mixture to the mixer bowl and mix on medium speed, 4 to 5 minutes. Scrape down the sides of the bowl, if necessary. Pour batter into the prepared pan.

Stir together reserved flour mixture, mini chocolate chips, English toffee bits, pecans, and remaining salt in a small bowl until well mixed. Sprinkle toffee streusel evenly over the cake batter.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 30 to 35 minutes. Cool in the pan on a rack, about 10 minutes. Serve warm or at room temperature, directly from the baking pan.