Place porcini mushrooms into a medium heatproof bowl and cover with hot water. Set aside to soak for 20 minutes or until soft. Drain, reserving the liquid.
Heat 2 tbs oil in a medium frying pan over medium heat. Add garlic and cook, stirring occasionally, for 2-3 minutes or until light golden. Stir in porcini mushrooms and cook for 5-6 minutes or until just tender. Transfer to a bowl, set aside to cool.
Increase heat to high, add remaining 1/4 cup oil and button mushrooms to the pan and cook, stirring often, for 8-10 minutes or until golden around the edges and tender. Set aside to cool, drain excess moisture.
Place the porcini, olives and capers into a food processor bowl. Pulse until finely chopped. Add the button mushrooms and pulse until mixture finely chopped (do not puree). Remove to a bowl. Add lemon juice, chives and salt, pepper. Stir in 1-2 tablespoons of the porcini liquid to adjust the consistency. Spoon into a medium sterilized jar. Drizzle with a thin layer of extra virgin olive oil to completely cover tapenade. Cover jar with a tea-towel and set aside to cool completely. Seal with lid and refrigerate for up to 2 months.
Serve with baked crostini or crackers.