Preheat oven to 180 and grease a 20 cm springform tin and line the base with baking paper. Place pretzels in a food processor and pulse to a fine crumb.
Add sugar and melted butter and pulse to combine. Press mixture into the bottom of the prepared tin pressing lightly into a even layer.
Bake for 10 minutes or just to golden brown on the edges. Remove from oven and cool on a wire rack.
Reduce oven to 160. In a large mixing bowl beat the cream cheese and sugar until smooth. Add lemon juice and vanilla and beat to combine.
Add egg yolks and beat on low speed just until combined. In a small bowl beat egg whites on high until stiff peaks form then gently fold into the cream cheese mixture.
Pour over the crust and place the tin on a baking sheet and bake in the oven at the reduced temperature for 25-30 minutes or until the center is almost set. Cool in tin on a wire rack for 1 hour then chill in the refrigerator until completely cooled.
In a microwave in 30 second bursts melt chocolate and butter and stir it until it gets smooth. Set aside to cool. In the meantime place the egg yolks and icing sugar in a heatproof bowl.
Place bowl over a small saucepan of simmering water and whisk the mixture continuously or it will turn into scrambled eggs until it reaches 160 or for about 10 minutes the mixture is thick and smooth.
Whisk in the chocolate mixture. Set bowl and ice and stir until cooled about 2 minutes. Fold in whipped cream. Spread over cheesecake and smooth with a spatula.
Sprinkle m&m's on the top to decorate then refrigerate until it is set. Remove sides of pan before serving. Can be stored in the refrigerator for several days.