Preheat the oven to Gas Mark 4, 180°C, fan 160°C. Butter 2 x 20cm (8in) loose-bottomed tins and line with nonstick baking paper. Beat the butter and sugar with an electric hand whisk for about 5 minutes or until light and fluffy. Add the eggs, one at a time, and beat well after each addition. Then fold in the flour and baking powder using a large metal spoon. Mix the coffee powder with 1 tablespoon of hot water and fold into the cake mix with the chopped pecans. Divide the cake mix between the two tins and smooth the surface. Bake for 20 minutes or until a skewer inserted into the middle of the cake comes out clean and the surface of the cake is slightly springy to touch. Remove from the oven, leave to cool for 5 minutes in the tin and then turn out onto a wire rack and leave to cool completely. Drizzle the flat side of one cake with a tablespoon of coffee liqueur. To make the icing, beat the mascarpone together with the sugar and the rest of the coffee liqueur until smooth. Sandwich the cooled cakes with a third of the filling and then cover the top and sides of the cake with the rest of the filling. Decorate with the pecan halves and a little pile of chocolate-covered coffee beans in the middle. This pudding is very rich and indulgent, so shouldn't be eaten too often. But it's perfect for those special occasions!