Preheat the oven to 350F.
In a double boiler or bowl over pot with simmering water melt butter and semisweet chocolate. Remove from heat and let cool 5-10 minutes. With a mortar and pestle or spice grinder grind your lavender until fine.
In a second bowl mix together your sugar, eggs and vanilla. Stir in the white chocolate and lavender. Add your flour and salt.
Line an 8 inch greased baking pan with parchment or foil and ensure it extends over opposite edges. Add batter and bake for 25 minutes or until tester comes out clean. Let cool in pan.