Preheat oven to 350°F.
Place flour, sugar, baking powder, baking soda, and salt in the bowl of a food processor fitted with a steel blade and pulse 1-2 times to mix.
Add zest of one lemon, the larger amount of calendula blossoms and cold cubed butter.
Pulse several times until mixture is crumbly.
Add lemon juice, half and half, and vanilla, pulsing again just until it comes together.
Turn out onto a silpat lined baking sheet and knead a few times to bring it into a smooth ball.
Pat into a round disk 8-inches by 1/2-inch thick.
Score with a knife into six wedges.
Sprinkle with topping and pat it down to make it stick.
Bake for 25 minutes.
Remove from oven and using a chef's knife, finish cutting through the score marks and separate the pieces on the pan so air can reach the sides.
Bake five minutes more.