With an electric mixer, mix butter and sugar until fluffy. Add the eggs, one at the time, and stir well.
Sift the flour and baking powder together, mix some of it onto the batter, then add some of the milk, then some of the batter until all is blended. Mix as little as possible.
Smear half of the batter in a buttered spring form ø23 cm.
Peel and take out the core of the pears, cut in pieces, and spread out on the cake batter. Sprinkle with 1 teaspoon of cinnamon, and cover with the rest of the batter.
Place a new layer of pear pieces on top; push them a little down into the batter. Sprinkle with the remaining cinnamon and Demerara sugar.
Bake in oven at 180°C for 30-45 minutes, depending on the amount of pears used(many pears - more moist - more time). The cake is ready when a wooden toothpick comes out clean.