In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a large mixing bowl, beat the softened butter and granulated sugar on medium-high speed for 2 minutes until light and fluffy. Add the egg and vanilla extract, beating until well combined.

Turn the mixer to low speed and gradually add the dry flour mixture in three additions, mixing just until a dough forms. Do not overmix.

Divide the dough in half. Roll each half out between two sheets of parchment paper to a 1/4-inch thickness. Transfer the rolled dough (still between parchment) to a baking sheet and chill in the refrigerator for at least 1 hour.

Preheat oven to 350°F (175°C). Line baking sheets with parchment paper. Remove chilled dough, peel off top parchment, and cut out bunny shapes. Place shapes on prepared baking sheets 1 inch apart.

Bake for 8-10 minutes until the edges are just barely golden. Let cool on the pan for 5 minutes before transferring to a wire rack to cool completely.

Make the buttercream: Beat softened butter until creamy. Gradually add powdered sugar, 1 tablespoon of cream, vanilla, and salt. Beat until fluffy. Divide into bowls and tint with pastel gel food coloring.

Spread a layer of buttercream onto a cooled cookie. Immediately press the frosted side gently into a bowl of matching colored sanding sugar. Repeat with all cookies.

Use black decorating icing to pipe small dots for eyes and a nose onto each bunny. Let icing set before storing or serving.