Brownies: Lightly grease a 9 x 13-inch pan. Over medium-low heat, or in the microwave, melt together the butter and chocolate, stirring till smooth. Stir in the cocoa, espresso, vanilla, sugar, and salt. Whisk in the eggs one at a time. In a separate bowl, whisk together the baking powder and flour, then stir these dry ingredients into the chocolate mixture until just blended. Spoon the batter into the prepared pan, and bake the brownies in a preheated 350ºF oven for 34 to 38 minutes until their top is shiny but the middle is still soft. Remove them from the oven, and cool for 30 minutes before icing. Icing: In a medium-sized bowl, mix the instant espresso and hot water, then stir in the sugar and heavy cream or milk. Drizzle the icing over the brownies then sprinkle with the toffee bits (crushed), if you like. Cool the brownies for several hours before cutting into squares. Makes 24 brownies.