2) Whisk the flour, baking powder and salt in a bowl.
3) Beat the butter and sugar in a large bowl, with an electric mixer, at medium-high speed until creamy, about 2 mins. Scrape down the sides and add the egg and vanilla essence. Beat well, and then scrape down the sides.
4) While mixing on low speed, add the dry ingredients in 3 batches. Scrape down the bowl between each addition, and mix until just blended.
5) Transfer the dough to the prepared pan and spread evenly with a spatula. Bake until light golden brown with spots of darker golden brown and darker edges, about 30 mins. Cool completely, about 45 mins.
For the fruit:
1) Whisk the cornflour and sugar together in a small saucepan. Whisk in orange juice until mixture dissolves. Stir in the fruits and crystallised orange peel. Bring to a simmer over medium, stirring often; continue to cook, stirring until thickened into a compote, about 3 mins more. You should be able to draw the spoon along the bottom of pan and see the pan with no liquid seeping back. Remove from heat and transfer to a bowl. Stir in the nuts and lemon zest.
2) Spread fruit evenly over the top of the cooled crust.
3) Heat oven to 180C/Gas 4. Bake mazurkas until golden brown around edges and fruit sets but is still slightly glossy and jewel-like, about 10 mins. Cool in pan on rack.
4) Cut, using an oiled knife, into 24 bars. Serve.
Busy baker's tips: When double-wrapped, the cookie base keeps at room temperature for 3 days or frozen for 1 month. The fruit compote can be made up to 1 week ahead and refrigerated. Store completed cookies in an airtight container, layers separated with parchment paper, for up to 5 days.