Preheat oven to 350 degrees.
Sift together flour, sugar, baking powder, and salt.
Mix cream with egg, lemon zest and rosemary and allow to sit for 10 to 15 minutes to steep.
Use a pastry cutter or two knives to cut the butter pieces into the flour. Keep going until mixture resembles crumbs. Mix wet mixture with flour mixture; stir gently with a fork until combined. Mix should be crumbly, but if it's too crumbly to work with, splash in a small amount of heavy cream.
Turn dough onto a floured surface and lightly press it together until it forms a rough rectangle. Use a rolling pin to roll into a rectangle about 1/2 inch to 3/4 inch thick. Use your hands to help with the forming if necessary. Final rectangle should be about 18 inches by about 10 inches. Use a knife to trim into a symmetrical rectangle, then cut the rectangle into 12 symmetrical squares/rectangles.
Next, cut each square/rectangle in half diagonally, to form smaller triangles.
Transfer to a parchment or baking mat-lined cookie sheet and bake for 18 minutes, removing from the oven just before they start to turn golden. Allow to cool for 15 minutes on the cookie sheet, then transfer to a cooling rack to cool completely.