Preheat your oven to 350°F (175°C) and prepare an 8×8-inch baking pan by greasing or lining it with parchment paper.

Melt the unsalted butter in a saucepan over low heat, ensuring it is melted but not bubbling. Remove from heat and let cool slightly.

Combine the melted butter with the brown sugar and granulated sugar in a mixing bowl, stirring until smooth.

Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

In a separate bowl, whisk together the all-purpose flour, baking powder, and espresso powder.

Gradually add the dry mixture to the wet mixture, folding gently until just combined.

Fold in the toffee bits gently until evenly distributed throughout the batter.

Spread the batter into the prepared pan and sprinkle sea salt flakes on top. Bake for 20-25 minutes until edges are golden brown.

Allow the blondies to cool slightly in the pan, then cut into squares and serve warm or at room temperature.