In a mixing bowl beat butter with an electric mixer on medium to high speed 30 seconds. Add brown sugar, soda, and salt. Beat until combined. Beat in egg, maple syrup, and vanilla. Beat in whole wheat flour and as much of the all-purpose flour as you can with the mixer. Stir in remaining flour. Divide dough in half. Cover; chill 3 hours or until easy to handle.
On a lightly floured surface, roll each portion of dough to 1/8-inch thickness. Using 2-1/2- to 3-inch leaf cookie cutters dipped into flour, cut into leaf shapes. Place 1 inch apart on an ungreased cookie sheet. Bake in a 375 degrees oven 7 to 8 minutes or until edges are firm. Remove from cookie sheet and cool on a wire rack. Frost with Maple Frosting. Let frosting set. Place in an airtight container; store at room temperature up to 3 days. Makes about 54.
Beat together powdered sugar and butter. Beat in maple syrup to make easy to spread.