Preheat the oven to 200C/180C Fan/Gas 6. Line an 8 inch square baking tin.
Add 2 tbsp boiling water to the 2 tsp instant coffee to dissolve it. Set aside.
Combine the melted butter, granulated sugar and light brown sugar in a large mixing bowl or the bowl of a stand mixer. Combine on high speed for 3-5 minutes. Add the eggs and combine again. Then add the coffee and mix.
Fold in the flour, baking powder, white chocolate and pecans until a smooth batter forms. Pour this into the lined tin.
Bake for 30-35 minutes, until golden on top. Let cool on a wire rack.
Optional: Melt 25g white chocolate and drizzle over, adding some crushed pecans for decoration. Slice and serve.