Special equipment: One 10 x 15-inch jelly-roll pan, buttered
Set a rack at the middle level of the oven and preheat to 350 degrees.
For the cake batter, in a large mixing bowl, beat the butter and sugar until they're soft and light. Add the 3 eggs one at a time, beating well after each addition. Beat in the vanilla. In another bowl, stir the yolks into the milk. Sift together the flour and baking powder and stir into the batter in 3 additions, alternating with the yolks and milk, beginning and ending with the flour. Spread the batter in the prepared pan.
Scatter the blueberries evenly over the batter, and press them in gently.
To make the crumbs, mix the dry ingredients in a bowl. Stir in the melted butter. Rub the mixture to coarse crumbs by hand. Scatter the crumbs over the berries as evenly as possible.
Bake the cake for 50 to 60 minutes, until the batter is firm and the crumbs are well-colored.
Cool the cake in the pan on a rack. Cut the cooled cake into fifteen 3-inch squares. Remove the squares from the pan to a platter. Serve the cake squares as a casual dessert--they're great for a picnic. Store loosely covered at room temperature.