Preheat oven to 350 degrees. Line a large baking sheet with parchment paper. Set aside.

In a large bowl whisk together flour and baking powder. Use a spoon to measure flour into measuring cup and level with a knife so as not to over measure.

In the bowl of stand mixer, cream butter and sugar until fluffy. Add lemon zest, vanilla and egg and continue to beat until well blended.

Add the flour mixture to the butter mixture one cup at a time until incorporated.

If dough appears dry, wet your hands with water to moisten and then gather in a ball.

Turn dough out onto a lightly floured surface. Separate into three smaller portions. Roll dough out to 1/4 inch thickness and cut with cookie cutters.

Transfer to prepared sheet and bake for 10-12 minutes or until lightly browned. Let cool on sheet until firm then remove to wire rack to cool completely before frosting with royal icing.

Combine all ingredients for royal icing in the bowl of a stand mixer fitted with whisk attachment. Mix on low for one minute then high for two minutes.

Transfer about 2 cups to a small bowl. Spray mixture with some water to thin out somewhat before adding gel food colors.

For complete step by step video instruction click on link above to go to Meghan's video for marbling the cookies.