In mixer bowl, cream butter and sugar together until light and fluffy. Add salt, lavender oil and egg; beat well. Sift flour and baking powder together; add to mixer bowl and mix on slow, stopping as soon as flour is incorporated. Mix in raisins. Turn dough onto lightly floured surface; divide into four equal pieces and shape into long loaves and 1 1/2-inches square. Wrap in plastic wrap and chill until solid. 

Preheat oven to 375°F. Then, working with one loaf at a time, brush outside with egg wash and roll in granulated sugar. Cut each loaf crosswise into about 18 thin 1 1/2-inch squares. Arrange on parchment-lined baking pan and bake at 375°F for 8 to 10 minutes or until edges are firm and bottoms are lightly browned.