Preheat oven to 350°F / 180°C / Gas mark 4. Adjust oven rack to the middle of the oven.
Make the crumb topping
In a small bowl, mix sugars together with nutmeg (or cinnamon) and salt.
Stir in melted butter.
Using a fork, gently mix in flour, making sure to scrape along the bottom of the bowl as you are mixing to incorporate sugar mixture.
Do not over mix - otherwise, you will not have big pieces of crumb topping. Set aside.
Make the cherry crumb cake
Butter and flour an 8 x 8 glass baking dish or cake pan. Make a sling with parchment paper, if desired, by placing two overlapping strips of parchment in the bottom of the dish.
In a medium bowl, combine flour, baking powder, baking soda and salt. Whisk to combine.
In a separate medium bowl, cream butter and sugar together until light and fluffy, about 3 minutes.
Add eggs, one at a time, fully incorporating egg before adding the second one, then beating to fully incorporate again.
Mix the vanilla and sour cream in and beat to combine.
Add the flour mixture and beat until just combined. A few small pockets of flour are ok.
Gently fold cherries into batter. Spoon batter into the baking dish and use an offset spatula if necessary to level the top.
Sprinkle the crumb topping evenly over the entire cake.
Bake crumb cake for 50 - 65 minutes, or until a tester comes out clean with a few moist crumbs.
Cool on wire rack before slicing.