Prepare the topping: Combine plums and liqueur in a small saucepan over medium, and cook, stirring occasionally, until only about 1 tablespoon of liquid is remaining, and plums are plumped, about 10 minutes. Add butter, and cook, stirring often, until melted. Whisk in brown sugar. Pour mixture into a lightly greased 9-inch square cake pan, and set aside.
Prepare the cake: Preheat oven to 350°F. Beat butter in bowl of an electric stand mixer fitted with the paddle attachment on high speed until light and fluffy. Add sugar and vanilla, and beat until combined, 2 to 3 minutes. Add eggs, 1 at a time, beating until combined after each addition. Add orange zest, and beat just until combined.
Sift together flour, baking powder, and salt. Alternate adding flour mixture and milk to batter, and beat until incorporated after each addition.
Spoon batter over topping in prepared cake pan, spreading gently. Bake in preheated oven until cake is golden and slightly pulling away from sides, 40 to 45 minutes.
Cool in pan 15 minutes, and invert cake onto a platter. Serve with Crème Fraîche Ice Cream.
The Walrus and the Carpenter, plus others, Seattle