Preheat oven to 375°F with rack in middle. Generously butter a 6inch round cake pan. Line bottom with a round of parchment paper and butter parchment.

In a bowl mix sugar and vanilla extract and set aside

Pulse cranberries with ¼ cup vanilla sugar in processor until coarsely chopped (do not purée).

Whisk together 1 cups flour, baking powder, and salt.

Beat together ½ stick butter and ½ cup vanilla sugar in a bowl with an electric mixer at medium-high speed until pale and fluffy. Add egg beating well after each addition. Scrape down side and bottom of bowl. Reduce speed to low and mix in flour mixture and milk alternately in batches, beginning and ending with flour, until just combined.

Spread half of batter in pan, then spoon cranberries over it, leaving a ½-inch border around edge. Top with remaining batter and smooth top.

Blend remaining ⅛ cup vanilla sugar with remaining tablespoon each of butter and flour using your fingertips. Crumble over top of cake.

Bake until a wooden pick inserted into cake (not into cranberry filling) comes out clean and side begins to pull away from pan, 45 to 50 minutes. Cool in pan 30 minutes, then remove from pan and cool completely, serve.